Shopping for a variac

[13hm13]

I'm in the USA, so 110-130V is my mains.

Looking for a good reliable variac. Cost should be under $100 USD.
10-20 amps is okay (max. required is enough power for 200w CRT based tv)

New or used is fine-- even classic/vintage in good shape. Made in China okay if the quality/reliability is decent.
I'd rather it be shipped from within USA due to Coronavirus shipping delays from Asia.
eBay or Amazon are okay. AliExpress -- no!
Suggest some options.
Thx!

[bob91343]

I am not up to date on prices for variacs but my guess is that you need to raise your budget if you are looking for something new.

A 5A unit might be more appropriate.  I can't imagine a tv set drawing more than that unless you go back to the 1940s.

[Electro Detective]

Get a good working one from a local seller via Ebay etc

Avoid those suss shiny red widow makers, unless you're already familiar with variac internals
and game to open one up and give it a good check over and de-rate it under heavy load

fwiw varying AC with a load is something to pay once cry once,
and go cheap on other stuff 

Good luck, hope you score a goodie 

[Electro Fan]

Contact and call the owner to discuss variacs, specs, features, reliability, prices, etc.

https://www.ebay.com/itm/USA-STACO-3PN1010-TUBE-AUDIO-POWER-AMP-AMPLIFIER-HAM-LINEAR-ANTIQUE-RADIO-VARIAC/192271396348?hash=item2cc444adfc:g:tRwAAOSwvD5Z69Ta

[13hm13]

Contact and call the owner to discuss variacs, specs, features, reliability, prices, etc.

https://www.ebay.com/itm/USA-STACO-3PN1010-TUBE-AUDIO-POWER-AMP-AMPLIFIER-HAM-LINEAR-ANTIQUE-RADIO-VARIAC/192271396348?hash=item2cc444adfc:g:tRwAAOSwvD5Z69Ta

Nice ... but US $269.00!!! Didn't you read my OP
I was thinking more ....
https://www.amazon.com/gp/product/B076Y2XHRS/

YaeCCC Auto Voltage Transformer AC Variable Voltage Converter Transformer 110VAC Input, 0-130VAC (500W)

NOTE: It's only 5 amps, but I think this will be fine (my orig. thought was 10-20amps, but that, as I now see, is not needed.

[Ian.M]

That looks fairly cheap & nasty.   The most important thing to know is the load current so you should have an ammeter on your Variac, as overloading one is *BAD* and  usually expensive if you don't react quickly enough to cut power or back off on the knob.   The voltage readout a cheap meter like that gives you is really no better that what you already know from the knob position and your supply voltage, so the meter is essentially worthless.   If you do get one, treat it as a Variac 'kit', open it up and check all the wiring, and ask here if anything seems odd before putting it into service.

[Shock]

Powerstat 136B or 236B

High quality, reversible percentage/voltage face plate, can be horizontal or vertical mounted, in or outside of an enclosure. Seperate voltage taps and can have custom socket enclosure mounted over the connections. You should be able to get replacement brushes for them as well.

Remember with large transformers etc you have inrush current which can trip your mains if you have fast acting circuit breakers. So you may need a soft start solution depending on what you are doing.

[Gyro]

Whatever you buy, make sure you modify it to add a fuse on the OUTPUT.  When the output voltage set low, it's very possible to burn out the winding / wiper contact without getting anywhere near blowing the input fuse or breaker.

[drussell]

Looking for a good reliable variac. Cost should be under $100 USD.

I don't think those two things are compatible. 

If you find a new one for under $100, it will be pretty much complete junk, though it may be sufficient as a starter model for your purposes if operated with care.  A good quality used one, in working condition, will usually set you back more than that also, but you end up with a much more robust, reliable piece of gear that will likely last a lifetime unless it is abused.

YMMV, and good luck. 

[Electro Fan]

Contact and call the owner to discuss variacs, specs, features, reliability, prices, etc.

https://www.ebay.com/itm/USA-STACO-3PN1010-TUBE-AUDIO-POWER-AMP-AMPLIFIER-HAM-LINEAR-ANTIQUE-RADIO-VARIAC/192271396348?hash=item2cc444adfc:g:tRwAAOSwvD5Z69Ta

Nice ... but US $269.00!!! Didn't you read my OP
I was thinking more ....
https://www.amazon.com/gp/product/B076Y2XHRS/

YaeCCC Auto Voltage Transformer AC Variable Voltage Converter Transformer 110VAC Input, 0-130VAC (500W)

NOTE: It's only 5 amps, but I think this will be fine (my orig. thought was 10-20amps, but that, as I now see, is not needed.

I did see your original post with your price target.  I think it’s always good to be careful not to push someone beyond their budget as that is an important personal decision.  At the same time if someone asked for advice on a reliable/safe car and I thought their budget might be half or less of what it might take to get a reliable/safe car I might try to explain why it’s possibly a risk.  But rather than me explain it I thought you might benefit from a phone call with someone who I believe has the expertise to help you see the possibilities.  I bought a variac from the guy who owns the business in the link years ago.  I have run it on a 15 amp circuit with a varying but fairly heavy load for days at a time over the years and so far so good.  No big deal, any good variac should do likewise.  My variac has a meter for current with two sensitivities as well as a voltage meter.  Again, not a big deal.  The reason for the suggestion is that I remember speaking with the owner.  He could cite chapter and verse various General Radio models and the more or less the history of variacs and he offered to build or refurbish almost any type of variac I wanted.

Overall I found him to be willing to discuss variac designs at length but maybe most importantly in my book I found him to be both credible and trustworthy. Plus he was a super nice guy who delivered an excellent product that was exactly as he represented.  I think because he sells so many variacs of many configurations he can probably get the price down as low as anyone for a given level of functionality and reliability but he won’t sell you something unless he believes it’s safe.

Can’t hurt to call.  If his price is too high at least you will have heard whatever he has to say which might help you choose a variac from somewhere else.  Just my thoughts.  It’s your decision, of course.

[Electro Fan]

Fwiw, my variac looks a lot like this plus two meters and two fuses and it looked like it just came new (inside and out) from the factory.

https://www.ebay.com/p/1127332453?iid=372842692756

Call the owner in that eBay link above that I first posted above (I don't know anything about the unit or seller in this link just above in this post).


Edit: Ok, here you go
http://www.shitintogold.com/VARIAC.php
http://www.shitintogold.com/

Another Edit: Owner's name is David.  Looks like he might have a different/new URL; I think the company name was / maybe still is DRCO.  I might have picked a different URL but you're buying a variac not a PR firm.

Yet another edit:
Tell David you don't want (can't afford / can't justify) the brand new cosmetics - you just want his least expensive reliable variac and see what he says.  His pricing reflects the fact that he likes to make products that operate like and look like they came new from GenRad decades ago - but I think he can cut the price down if you go for the electronics over the case condition.  Tell him that you are going to report back to EEVblog if he takes good care of you (that's his only MO anyway) and therefore this is his chance to tap into a new market - but first you need a reliable variac that you can afford.  If his least expensive unit is too expensive for your budget keep shopping elsewhere - but I think he will try to take good care of you.

Good luck!

And another PS,

Those Powerstats like Shock posted look good.  Ask David if you go for a Powerstat vs. a GenRad if that would help keep the budget in line.  For all I know the Powerstats might be the same or more or less; the good thing with David is that he just likes making and refurbing good products and he's big into variacs.  I don't think he has any axe to grind whether it's Powerstat, GenRad, Staco, or any other manufacturer, model, or design.  It's like asking Michelangelo for something in marble - but it needs to be an affordable statue.

[Cubdriver]

Skip the cheap Chinese-made ones and go with old American iron.

Examples:
https://www.ebay.com/itm/Powerstat-S1138-Variac-Variable-Autotransformer-0-140V/162588872075?hash=item25db0d4d8b:g:3ZwAAOSwfIxZZodq
https://www.ebay.com/itm/4211-STACO-3PN1210-Variable-Autotransformer/143152811752?epid=27009075506&hash=item2154928ee8:g:eL8AAOSwfCZZ1pC-
https://www.ebay.com/itm/Superior-Electric-Powerstat-Type-3PN117B-Autotransformer-0-120-V-12-A/133080862801?epid=1781743689&hash=item1efc3cd051:g:XkYAAOSwRjlc~uAH:sc:USPSPriorityFlatRateBox!06795!US!-1

If you're willing to wire in the connections yourself:
https://www.ebay.com/itm/Superior-Electric-Company-Powerstat-Variable-Transformer-Type-116-0-140V-Output/324143800062?hash=item4b7879defe:g:~cYAAOSwvpZeoIjn

This one needs a cover for the connections:
https://www.ebay.com/itm/STACO-ENERGY-1210CT-VARIABLE-POWER-TRANSFORMER/192886776612?epid=8032348549&hash=item2ce8f2a324:g:IKQAAOSwUuNcvkf6

Just some options.  Good luck in your search.

-Pat

[BravoV]

Below is Matsunaga, well known intenational brand variac manufacturer for decades, and I've seen one at industrial site that is still working fine after 50 years. 

Its 1 KVA rated, bought it used, and did inspection tear down when I received it.

Here are few photos of it's internals.




The carbon wiper and exposed windings close up shots ...

[drussell]

Whatever you buy, make sure you modify it to add a fuse on the OUTPUT.  When the output voltage set low, it's very possible to burn out the winding / wiper contact without getting anywhere near blowing the input fuse or breaker.

Indeed.  A proper old-school variac is usually wired this way:



Obviously you could add an input fuse also on a smaller model or a cheap one that you don't trust the autotransformer windings on, but traditionally on larger ones they don't even bother with an input fuse.  That's what the branch circuit supply breaker is for.   

As for the prices, paying $100-150 or even more, for a quality used unit in good shape by the time you have it shipped, etc. is actually a bargain compared to some generic $60 Chinese unit in the long run.

There is a reason why something high-quality like a brand new production 10A  ISE "Variac", a Superior "Powerstat", or a STACO Energy variable autotransformer costs $750. 

[13hm13]

Great tips everyone!

I like the idea of buying a new eBay/Amazon China-made unit and modding it for better safety and reliability. I realize that if the Chinese variac is pretty crappy to begin with (cheap, low -quality coils, etc), modding it for the better may be impossible.
All the "red" China units (eBay or Amazon) seem to ultimately be sourced from a single source. And have generated sizeable and favorable Amazon reviews (for what those are worth).

[tkamiya]

Contact and call the owner to discuss variacs, specs, features, reliability, prices, etc.

https://www.ebay.com/itm/USA-STACO-3PN1010-TUBE-AUDIO-POWER-AMP-AMPLIFIER-HAM-LINEAR-ANTIQUE-RADIO-VARIAC/192271396348?hash=item2cc444adfc:g:tRwAAOSwvD5Z69Ta

Nice ... but US $269.00!!! Didn't you read my OP
I was thinking more ....
https://www.amazon.com/gp/product/B076Y2XHRS/

YaeCCC Auto Voltage Transformer AC Variable Voltage Converter Transformer 110VAC Input, 0-130VAC (500W)

NOTE: It's only 5 amps, but I think this will be fine (my orig. thought was 10-20amps, but that, as I now see, is not needed.

I have that exact same one, purchased from Amazon.

It's rated for 0.5KVA but based on size, weight, what I had in the past, and general knowledge in transformers, I wouldn't rate it at that.  Honestly, 2A maybe more reasonable limit.  The meter and the dial is way off, so be sure to have a DVM and check it before plugging anything in.  On spec, they mix up amps, watts, and VA.  They are very confusing and not to be trusted.  I also don't feel comfortable using it unattended. 

If I have to do this again, I'd buy a used one, either Japanese or American made.  You'd be surprised how big and heavy they are for given amperage. 

[drussell]

All the "red" China units (eBay or Amazon) seem to ultimately be sourced from a single source. And have generated sizeable and favorable Amazon reviews (for what those are worth).

They're not all red.

My local industrial electronics shop stocks everything from the $700 STACO to some cheap Chinese one that is only a slightly-more-robust version, painted beige instead of red, that came out of the same Chinese factory of your above-linked Amazon listing.

They also don't always get great reviews....

Unit received inoperative. It did not show any voltage on meter although power switch lit when unit turned on. Checked fuse and found it to be good. However, noticed that upon screwing fuse holder cover back onto fuse holder there was no back pressure to indicate that the fuse was seating properly. Removed two front screws, examined interior and found that fuse holder was broken. Having several spare fuse holders of this general type, replaced holder and soldered two wires back into place.

Horrible wiring. The solder joints in this device are terrible and the input live broke off the switch after about a month of use. Unaware of the loose wire, I plugged it in yesterday and was greeted with sparks and a pop. Thankfully the gfci outlet did its job. The pictures I included show where the short occured as well as the haphazard wiring my device was shipped with.

Also, the indicated voltage on both the reference dial and analog v-meter display voltages substantially below what the true output is. I can't remember the exact numbers but I was measuring something like 135V output when the device was set to 110V.

As other reviewers have mentioned, do not buy this device without a strong working electrical knowledge. This tool needs to be checked before use (which I failed to do) because there are obviously QC issues.

After taking the time to rewire everything with the appropriate terminals and resoldering, the problem with mine is fixed. However, because of the unsafe condition this was out of the box I reccomend spending the extra money on a safer, higher quality unit.

YMMV. 

[tkamiya]

Now, I have a sudden urge to do some destructive testing.  500 watts worth of incandescent light bulb as loads, instrument it and let it run.  (outside on concrete)

[drussell]

Now, I have a sudden urge to do some destructive testing.  500 watts worth of incandescent light bulb as loads, instrument it and let it run.  (outside on concrete)

Run it down from the top if you do, it will likely blow up at the bottom, probably even if you used an active constant current load instead of bulbs or heaters which will slowly become lower and lower resistance at lower and lower temperatures. 

Those little junk units aren't even rated for their actual load current and why they would specify a VA rating when you can't even get like 50 VA out at the bottom of the winding without ending up with what amounts to more of arc welding of the windings, I don't know... 

Take a look at something like the STACO brochure and you'll see that they rate them to do the full current even at the low end.

I doubt those Chinese units will fare so well above, like you say, perhaps 250VA?

And what about a rectifier-capacitor load?  With those tiny autotransformer windings?  Current peaks?!  No way.

Please post video. 

[Cubdriver]

Great tips everyone!

I like the idea of buying a new eBay/Amazon China-made unit and modding it for better safety and reliability. I realize that if the Chinese variac is pretty crappy to begin with (cheap, low -quality coils, etc), modding it for the better may be impossible.
All the "red" China units (eBay or Amazon) seem to ultimately be sourced from a single source. And have generated sizeable and favorable Amazon reviews (for what those are worth).

You are correct that a basic variac doesn't consist of much beyond the coils and brush.  Given that, if the heart of the device is crap, what do you expect to modify to make better?  I'll repeat my suggestion that you go with a used version of a major US or Japanese brand - they are well built, conservatively spec'd and very reliable if not horribly abused.  We got one of the red ones at work - needed to buy 'new' for a small project.  It's a hunk of junk - I much prefer my 30+ year old Superior Electric models.  I got one of them surplus when NSC closed their Danbury CT plant 30 years ago.  It was old then, and had been in service powering an adjustable heater in the dopant supply section of a diffusion furnace for probably 15-20 years at that point.  It's not pretty, but it works perfectly.

Buy once, cry once.

-Pat

[engrguy42]

Now, I have a sudden urge to do some destructive testing.  500 watts worth of incandescent light bulb as loads, instrument it and let it run.  (outside on concrete)

Now you're talkin'.  Nothing like actual data rather than vague generalizations.

I'm all for the "don't buy junk" motto, but I'm also mindful that in the tech world there's a whole lot of paranoia over stuff people maybe heard about something and made up their minds, but never actually tested it themselves. Y'know, like, say, ATX power supplies. 

Anyway, I'm also kinda wary of buying stuff on eBay that's obviously been used for decades. Yeah, that US-made stuff was probably built extremely well, but who the hell knows who used it, and on what?

As far as variacs, I'm also considering buying one maybe. And I'm trying to figure what actually can go wrong with them that make them "junk". Hell, it's a variable autotransformer. Is the wire insufficient to handle the current? The insulation insufficient? The only moving part is the brushes, so are they gonna fail? Maybe they get damaged in shipment since it's such a big heavy thing? Of course the built-in meter is probably inaccurate as hell, but that's why we have multimeters. And fuses are cheap.

I dunno, I'm a bit skeptical about the skepticism... 

[drussell]

And I'm trying to figure what actually can go wrong with them that make them "junk". Hell, it's a variable autotransformer. Is the wire insufficient to handle the current? The insulation insufficient? The only moving part is the brushes, so are they gonna fail?

Well, yes, it is just a variable autotransformer, so the windings could be burned, broken, shorted, etc.  Sometimes that can be mitigated by bypassing a couple of turns by removing and then shorting the wiper-contact-area of those turns so it just has an "uneven" spot instead of a total dead spot or open winding, or whatever...

Brushes?  Yes, the brush contact assembly can require careful refurbishment or outright replacement if they have been used heavily or overloaded repeatedly.  They are typically still available for even older models but expect to pay close to $100 for the complete replacement brush and holder assembly, though if not abused a new one should easily last another 50+ years in regular operation.

[engrguy42]

And I'm trying to figure what actually can go wrong with them that make them "junk". Hell, it's a variable autotransformer. Is the wire insufficient to handle the current? The insulation insufficient? The only moving part is the brushes, so are they gonna fail?

Well, yes, it is just a variable autotransformer, so the windings could be burned, broken, shorted, etc.  Sometimes that can be mitigated by bypassing a couple of turns by removing and then shorting the wiper-contact-area of those turns so it just has an "uneven" spot instead of a total dead spot or open winding, or whatever...

Brushes?  Yes, the brush contact assembly can require careful refurbishment or outright replacement if they have been used heavily or overloaded repeatedly.  They are typically still available for even older models but expect to pay close to $100 for the complete replacement brush and holder assembly, though if not abused a new one should easily last another 50+ years in regular operation.

Yeah, but what, specifically, about those cheap Chinese "junk" variacs is junk?

[tkamiya]

What specifically make it a junk?

It's too light.  That means the core (that wires are wound on) is too small.  It will saturate and overheat.  Wires are also too thin for the rating.  When I lived in Japan, I had a 250VA version.  It was larger and heavier than this thing.

My prediction is, it will get way too warm, possible smoke, possible broken wire, but I don't expect anything spectacular.  I think I have a 500 watt light bulb somewhere, so I'll use that as a load.  Thermo-couples on bottom, face plate, sides.  Clamp on meter for current at input, Voltage at the output.  Run it for one hour at half way point, and another one hour at whatever the bulb is rated.  (120V?)  Then rotate the slider from 0 to 120V back and forth few times.  This is when I expect something to go wrong. 

Breaking it isn't the point, and stressing until it melts isn't a point either.  My goal is to determine, if _I_ feel safe using this at the rating.  Obviously, at half way point, it won't be expending 500VA.  I am a licensed electrician but this is only a partial and personal test in home setting.  I don't endorse, approve, or guarantee anything and if anyone decides to buy it, they do so at their free will.

It might take me few days but I've been wanting to do this for a while, so I will do it.  Just don't stay up all night waiting for it. 

[Electro Fan]

And I'm trying to figure what actually can go wrong with them that make them "junk". Hell, it's a variable autotransformer. Is the wire insufficient to handle the current? The insulation insufficient? The only moving part is the brushes, so are they gonna fail?

Well, yes, it is just a variable autotransformer, so the windings could be burned, broken, shorted, etc.  Sometimes that can be mitigated by bypassing a couple of turns by removing and then shorting the wiper-contact-area of those turns so it just has an "uneven" spot instead of a total dead spot or open winding, or whatever...

Brushes?  Yes, the brush contact assembly can require careful refurbishment or outright replacement if they have been used heavily or overloaded repeatedly.  They are typically still available for even older models but expect to pay close to $100 for the complete replacement brush and holder assembly, though if not abused a new one should easily last another 50+ years in regular operation.

Yeah, but what, specifically, about those cheap Chinese "junk" variacs is junk?

Lots of people here know more about this than me but it's my understanding that, among other things, in a well made variac tolerances are tight (thousandth of an inch or better, better than you can usually get with a hand file), the compositions and  plated materials are made to avoid wear and corrosion, and generally the old school brushes were not dependent on (ie had no) heat sinks.

[engrguy42]

What specifically make it a junk?

It's too light.  That means the core (that wires are wound on) is too small.  It will saturate and overheat.  Wires are also too thin for the rating.  When I lived in Japan, I had a 250VA version.  It was larger and heavier than this thing.

My prediction is, it will get way too warm, possible smoke, possible broken wire, but I don't expect anything spectacular.  I think I have a 500 watt light bulb somewhere, so I'll use that as a load.  Thermo-couples on bottom, face plate, sides.  Clamp on meter for current at input, Voltage at the output.  Run it for one hour at half way point, and another one hour at whatever the bulb is rated.  (120V?)  Then rotate the slider from 0 to 120V back and forth few times.  This is when I expect something to go wrong. 

Breaking it isn't the point, and stressing until it melts isn't a point either.  My goal is to determine, if _I_ feel safe using this at the rating.  Obviously, at half way point, it won't be expending 500VA.  I am a licensed electrician but this is only a partial and personal test in home setting.  I don't endorse, approve, or guarantee anything and if anyone decides to buy it, they do so at their free will.

It might take me few days but I've been wanting to do this for a while, so I will do it.  Just don't stay up all night waiting for it. 

But for the iron core to saturate means the applied voltage is too high. So you're saying they can't actually handle 120 volts?? And are you sure the wire isn't rated for the current? What gauge do they use?

[Ian.M]

It will be the usual things for built down to a cost Chinese electrical gear:

* Not enough iron, so too close to saturation and prone to drawing excessive current and overheating if the incoming line voltage is higher than nominal.

* Not enough or low grade copper, so the winding resistance will be excessive and it will be unable to provide its rated current without overheating.

* Poor quality insulation, so a high probability of developing turn to turn or turn to core shorts with sustained use, especially at power levels that heat it significantly.

* Poor quality plastic parts, often of a plastic unsuitable for the application.   e.g. fuseholders that break in half due to the stress of the spring pressing on the fuse, control knobs that split, brush assemblies and terminal boards that disintegrate etc.

* Flimsy sheet metal work which wont hold fastenings reliably and distorts during normal handling.

* poor quality soft screws and nuts - good luck getting a good connection if tightening a terminal sheers it and there are few things more annoying  than a Variac control knob with too soft a grub screw that slips on the shaft if you try to turn it too quickly, or if you hit the end stop with too much force!

If you've got access to a thermal imaging camera, setting up to run it with the housing off or partially removed, and examining it for hot-spots running under load may be educational (and horrifying).

You can probably expect to get at most 50% of the max rated current without overloading a cheapo Variac, and may well have to do away with any boost capability completely by rewiring the input Line wire from the 'hot' end winding tap to the 'hot' end winding end to keep it out of saturation.  (The Neutral wire should always go direct to the 0V end of the winding.)

Edit:  three posts were made while I was typing this and there's quite a bit of duplication with my points.

[engrguy42]

Ian.M, again I'm trying to get down to actual facts. Is what you're describing speculation, or is it based on stuff you've witnessed with these variacs?

[tkamiya]

Magnetic core in transformer can saturate with excessive current.  (or said more precisely, as it approaches saturation)  It's not only the voltage that matters.  Please read what I said simply as "insufficient design."  I didn't run material analysis, flux density calculations, or measure wire gauges.

I should measure the wire size.  That's a good idea.

[tkamiya]

If anything, it will be fun. 

It will also be self-assuring ME to be using this.

If this was a local radio club or something, we'd all gather in back yard, do a BBQ, and run this test and have a good ol' fun.  Kind of hard to do that on Internet and during Covid virus pandemic. 

Like someone said, speculation and theorizing is one thing but real world test is quite another.  I'm a bit sad my spectrum analyzers, signal generators, frequency counters, and precise lab standards won't be part of this....

[engrguy42]

Magnetic core in transformer can saturate with excessive current.  (or said more precisely, as it approaches saturation)  It's not only the voltage that matters.  Please read what I said simply as "insufficient design."  I didn't run material analysis, flux density calculations, or measure wire gauges.

I should measure the wire size.  That's a good idea.

The magnetic core is a nonlinear I/V characteristic, analagous to a nonlinear resistance of, say, a metal oxide varistor. The reason excessive current would flow is due to the voltage getting up to the knee of the I/V curve into saturation territory. Above that the current increases quickly with small increases in voltage. And that current is comprised of odd(?) harmonics, which can cause heating and other bad stuff. So I'm not sure what you're referring to when you say it can saturate with excessive current. Excessive current with lots of harmonics is the RESULT of high voltage.

[Ian.M]

Ian.M, again I'm trying to get down to actual facts. Is what you're describing speculation, or is it based on stuff you've witnessed with these variacs?

Some I've witnessed (e.g. the slipping knob), some I've personally heard from other techs,  but much of the above is generalisations from watching the quality of electrical goods imported from China dropping year on year over the course of my career.

[engrguy42]

Ian.M, again I'm trying to get down to actual facts. Is what you're describing speculation, or is it based on stuff you've witnessed with these variacs?

Some I've witnessed (e.g. the slipping knob), some I've personally heard from other techs,  but much of the above is generalisations from watching the quality of electrical goods imported from China dropping year on year over the course of my career.

And I'm sure we could all point to some of our favorite, high quality equipment that is made in China.

[tkamiya]

Test jig.....

That light bulb is a 500 watts on its own. 

[drussell]

Magnetic core in transformer can saturate with excessive current.  (or said more precisely, as it approaches saturation)  It's not only the voltage that matters.  Please read what I said simply as "insufficient design."  I didn't run material analysis, flux density calculations, or measure wire gauges.

I should measure the wire size.  That's a good idea.

Indeed.  A critical analysis would be instructive. 

I expect they work fine though most of the range on an easy, simple, resistive load, especially intermittently but as soon as you try running motors or battery chargers or other horrible loads through them, like trying to troubleshoot an SMPS or something, they're useless...  I've been told by a reliable personal friend who is an EE that you can even see the obvious core saturation on current peaks on at least one particular example, although I have never actually attempted to verify this myself on my meager equipment stash and wouldn't spend my own money on one of those things just to test it, even though there are presumably several variations of that same design coming out of one factory with unknown, varying quality levels, as is customary for Chinese factory goods.

Personally I've seen a blown up Chinese model one in person, but I didn't blow it up. 

I've also used many, many old boat-anchor ones, including some complete rust buckets and don't recall ever having an actual problem or witnessing any fault in any one in actual operation including some chassis-mount ones in fairly harsh environments. 

I helped fix a couple of pieces of ones for a friend of mine years ago or else I never would have even seen the internal mechanics of the old ones, they normally just work.    They were "for repair/parts" type units with obvious smashing, breakage and general smooshy-smashage from a whole discarded box of them, but at the difficult-to-argue-with price of free, then  .-- .... -.--  .-- --- ..- .-.. -..  -.-- --- ..-  -. --- - ..--..

[engrguy42]

Test jig.....

That light bulb is a 500 watts on its own.

Nice.  I'm staying up all night waiting for your results. 

Are you done yet? 

[tkamiya]

Thing about Chinese, Japanese, Indian, whatever country's goods is that they make to the spec of whomever is ordering. 

I have two fabulous quality microscopes made in India, and piece of crap camera made in Japan.  This piece of crap auto-transformer is here because we, the buyers, demanded cheap products.  Some manufacturers will say "sorry, we can't make it for that", and for every one of those there are ten who will put something together and ship them.

I just have one question.  Who will hold my beer while I do this test?

[drussell]

And I'm sure we could all point to some of our favorite, high quality equipment that is made in China.

Don't get me wrong, there are actually some impressively high quality products that come out of China.

Certain products can end up being ridiculously good bargains overall, but those standard red-model Variac clones do not tend to be one of them, at least from what I've seen. 

Let the ritual of the smoke release commence! 

[engrguy42]

Thing about Chinese, Japanese, Indian, whatever country's goods is that they make to the spec of whomever is ordering. 

I have two fabulous quality microscopes made in India, and piece of crap camera made in Japan.  This piece of crap auto-transformer is here because we, the buyers, demanded cheap products.  Some manufacturers will say "sorry, we can't make it for that", and for every one of those there are ten who will put something together and ship them.

I just have one question.  Who will hold my beer while I do this test?

Don't forget the video 

[tkamiya]

I have no ability to shoot video or edit one.   I'll see what I can do but you might have to settle for a photo or two.

Dave?  Can you come to Orlando, Florida by tomorrow?

[engrguy42]

I have no ability to shoot video or edit one.   I'll see what I can do but you might have to settle for a photo or two.

Dave?  Can you come to Orlando, Florida by tomorrow?

Reminds me of a computer tech youtuber I used to watch (before he decided to pander to the mindless entertainment crowd). In the computer world people turn purple and pee in their pants over stuff like CPU cooling, and stuff they actually don't understand. "OHMYGOD, you need to have tons of cooling or the whole computer will melt!!!".

So he'd do stuff like take an old computer, yank all CPU cooling, and turn it on and see what happens. And of course it throttled the CPU and kept below damaging 100C temps, just like the engineers designed. Or he'd place a motherboard in the sink, under water, then plug it in and see what happens. Stuff like that where you actually get beyond the hype and paranoia and get to the ACTUAL facts. I wish more folks would do that. (wonder if Dave is listening).

Anyway...tkamiya you're doing a great public service    Thanks much.

Are you done yet ?? 

[drussell]

I have no ability to shoot video or edit one.   I'll see what I can do but you might have to settle for a photo or two.

Dave?  Can you come to Orlando, Florida by tomorrow?

Editing in a pinch is best just with virtualdub.  Easy peasy.  It's basically usually just one .exe file. 

Any phone, camera, anything with video capability?  Even a 640x480 potato like mine? 

(...although I don't actually expect much excitement with a light bulb...)

[tkamiya]

I think, all of us who are at or above middle age will understand this.  It's amazing we lived this far and through our own stupidity.  All those chemistry sets and managzines for science of all kinds, they included instructions and we actually did them.  We didn't watch it on You-tube, and there weren't adults telling us not to. 

Then we got creative.  Myself, I've tried to charge 1.5V battery by directly plugging into mains and see if I can "super charge" it.  Tried to catch a falling soldering iron and ended up grabbing the hot end.  Forgot to discharge capacitor and ZAP!  Find a tantalum cap and stick it into mains outlet (that one hurt quite a bit)

Somehow, the world was safer with us "scientists" running lose.

[tkamiya]

I found a D800.  I never used it for a video so I'll have to figure this thing out.

[Electro Fan]

I found a D800.  I never used it for a video so I'll have to figure this thing out.

Good find

[Electro Detective]

I think, all of us who are at or above middle age will understand this.  It's amazing we lived this far and through our own stupidity.  All those chemistry sets and managzines for science of all kinds, they included instructions and we actually did them.  We didn't watch it on You-tube, and there weren't adults telling us not to. 

Then we got creative.  Myself, I've tried to charge 1.5V battery by directly plugging into mains and see if I can "super charge" it.  Tried to catch a falling soldering iron and ended up grabbing the hot end.  Forgot to discharge capacitor and ZAP!  Find a tantalum cap and stick it into mains outlet (that one hurt quite a bit)

Somehow, the world was safer with us "scientists" running loose.

What evidence is there it's not still going on ?    

 

[drussell]

I found a D800.  I never used it for a video so I'll have to figure this thing out.

Good find

Indeed!   

I wish I could randomly find things like that around here!

[Electro Fan]

I found a D800.  I never used it for a video so I'll have to figure this thing out.

Good find

Indeed!   

I wish I could randomly find things like that around here!

This might be a brother to the "What did you buy today?" thread.... "What did you randomly find today?"

I'm guessing if the posts were ranked by good finds a D800 would be on the upper half of the list

[bdunham7]

I'm in the USA, so 110-130V is my mains.

Looking for a good reliable variac. Cost should be under $100 USD.
10-20 amps is okay (max. required is enough power for 200w CRT based tv)

New or used is fine-- even classic/vintage in good shape. Made in China okay if the quality/reliability is decent.
I'd rather it be shipped from within USA due to Coronavirus shipping delays from Asia.
eBay or Amazon are okay. AliExpress -- no!
Suggest some options.
Thx!

If you're working on old TVs and the like, try to find a BK Precision 1655 or 1655A (the older one is actually better, IMO) which will have 3A/4A peak and isolation, leak testing and an input circuit breaker.  Output protection is by fuse.  I got one with some minor, fixable cosmetic damage on eBay for $85, although I don't see any on there now anywhere near that price.  EDIT:  The BK 1653 seems to go for prices near your budget.  It is only 2A/3A peak, but if that's enough, it's a good unit.

If you get the Chinesium model, let us know how it goes. I've seen them at Harbor Freight and I wasn't tempted to let one follow me home.

[tkamiya]

I took covers off mine and made some photo and measurements.

Bare wire thickness is 0.505mm
With enamel coating it is 0.573mm
(both measured by Mitutoyo Micrometer)

That means AWG 24 equivalent? 

Also, the sliding contact does NOT have a heat sink (it's only a 5A version) and it's a cone shaped carbon piece, so it makes a point contact; maybe crossing two windings?  Most solder joints were cold soldered and it did not look right, but I have no ways to further analyze this.  Lead wire seems to be vinyl insulated wires so they are not high temperature rated.  I'm saying it's vinyl by feel of consistency and the fact at every (cold) solder joint, insulation has charred black.

I'll withhold my opinion until the testing is done.

[bdunham7]

I took covers off mine and made some photo and measurements.

Bare wire thickness is 0.505mm
With enamel coating it is 0.573mm
(both measured by Mitutoyo Micrometer)

That means AWG 24 equivalent? 

So the wire is good for 1 amp--if it is copper.  After you are done blowing it up, can you check and see if the wire is copper or CCA?

[Cubdriver]

I took covers off mine and made some photo and measurements.

Bare wire thickness is 0.505mm
With enamel coating it is 0.573mm
(both measured by Mitutoyo Micrometer)

That means AWG 24 equivalent? 

Also, the sliding contact does NOT have a heat sink (it's only a 5A version) and it's a cone shaped carbon piece, so it makes a point contact; maybe crossing two windings?  Most solder joints were cold soldered and it did not look right, but I have no ways to further analyze this.  Lead wire seems to be vinyl insulated wires so they are not high temperature rated.  I'm saying it's vinyl by feel of consistency and the fact at every (cold) solder joint, insulation has charred black.

I'll withhold my opinion until the testing is done.

Wow - that's a 5 A unit?  That brush looks tiny compared to the one on a 3A, 0-140V Powerstat I just dug out.









-Pat

[tkamiya]

Hahaha.... 

I'd be laughing so hard if it's CCA.  But it isn't.  (read on...) 

Scarcely thing is, the contact is a POINT, not a surface.  Another thing is, see how your windings are plated with something?  Mine is just bare copper ground about third of a way down....  so the actual contact area and wire thickness is smaller than it appears.  The size of point will, at maximum, touch, but not fully cover 2 windigs. 

I'm actually thinking leaving the cover off and focus on inside.  I can point the infrared thermometer that way and if something interesting happens, it will be recorded.  It might get a bit exciting. 

I'm trying to be fair minded and not make judgement before the test but I do believe some pre-caution will be necessary.  Likely endings are fairly obvious.

[Cubdriver]

Shame you don't have an IR camera - that might make for an interesting bit of evaluation.  I seem to have misplaced mine on my last CA trip - am hoping I left it in the office out there, but that's a somewhat dim hope.  The downside of the tiny FLIR-1 it's easy to lose the damned thing!!

And FWIW, I got my dial caliper out and the wire on the 3A Powerstat appears to be #24 as well, FWIW. I suppose on a variac they can get away with less derating since it's a single layer winding and it can thus dissipate heat better than a conventional transformer with many layers on a bobbin would be able to.

-Pat

[drussell]

If you're working on old TVs and the like, try to find a BK Precision 1655 or 1655A (the older one is actually better, IMO) which will have 3A/4A peak and isolation, leak testing and an input circuit breaker.  Output protection is by fuse.  I got one with some minor, fixable cosmetic damage on eBay for $85, although I don't see any on there now anywhere near that price.  EDIT:  The BK 1653 seems to go for prices near your budget.  It is only 2A/3A peak, but if that's enough, it's a good unit.

The Sencore PR57 (or the PR570 descendant) is another good TV-service-bench specially designed, isolating type unit to look out for if you happen to find one for a reasonable price. 

The current asking prices on theBay are insane.

[tkamiya]

I am now looking for a new Variac.  I have a local surplus shop for that sort of things so I'll have to pay it a visit, after covid thing settles down.  It's one of those things I rarely use, but when I need it, there is no substitute. I've also been wanting isolation transformer.  Should I look for shiny new one from China on Amazon?   

I actually knew I had a D800 but I didn't associate it with using it for video.  It can be done but some features are quite lacking, like not being able to auto-focus during shooting.  Just few weeks ago, I was thinking of re-re-restarting film photography.

[Cubdriver]

I am now looking for a new Variac.  I have a local surplus shop for that sort of things so I'll have to pay it a visit, after covid thing settles down.  It's one of those things I rarely use, but when I need it, there is no substitute. I've also been wanting isolation transformer.  Should I look for shiny new one from China on Amazon?  

I actually knew I had a D800 but I didn't associate it with using it for video.  It can be done but some features are quite lacking, like not being able to auto-focus during shooting.  Just few weeks ago, I was thinking of re-re-restarting film photography.

 

Sure, go for it!   

-Pat

[Cubdriver]

If you're working on old TVs and the like, try to find a BK Precision 1655 or 1655A (the older one is actually better, IMO) which will have 3A/4A peak and isolation, leak testing and an input circuit breaker.  Output protection is by fuse.  I got one with some minor, fixable cosmetic damage on eBay for $85, although I don't see any on there now anywhere near that price.  EDIT:  The BK 1653 seems to go for prices near your budget.  It is only 2A/3A peak, but if that's enough, it's a good unit.

The Sencore PR57 (or the PR570 descendant) is another good TV-service-bench specially designed, isolating type unit to look out for if you happen to find one for a reasonable price. 

The current asking prices on theBay are insane.

Damn!!  They have gotten a bit insane, haven't they?  I got one of the 570s a few years ago after seeing it on Shahriar's channel, and love it for powering things up for the first time - go slow and watch things and shut down if they look wrong.  Thanks for prompting me to look, though - bought a new front panel overlay to replace the buggered up one on mine.

-Pat

[drussell]

Also, the sliding contact does NOT have a heat sink (it's only a 5A version) and it's a cone shaped carbon piece, so it makes a point contact; maybe crossing two windings?

Wow - that's a 5 A unit?  That brush looks tiny compared to the one on a 3A, 0-140V Powerstat I just dug out.

Yeah, that tiny brush contact point is anemic.  I don't remember the one I saw the inside of being a little cone like that, but I only looked at it to laugh at the charred remnants of the windings for a moment, several years ago... 

I always think of Variac brushes looking more like these:



A reasonable selection of replacements for the typical old ones are available here, by the way:

http://www.carbonbrush.com/variacbrush.htm

[drussell]

Another thing is, see how your windings are plated with something?  Mine is just bare copper ground about third of a way down....  so the actual contact area and wire thickness is smaller than it appears.  The size of point will, at maximum, touch, but not fully cover 2 windigs.

A properly made Variac winding will usually have the contact tops of the windings pressed down flat to maintain most of the cross-sectional area, not just filed off halfway through.     That's where the one Chinesium-grade one that I saw had blown up.  Cooked the tops right off of a bunch of windings.

[BravoV]

Yeah, that tiny brush contact point is anemic.

Yeah, really small contact area.

Found another photo of my Matsunaga teardown attached below.

[BravoV]

Another thing is, see how your windings are plated with something?  Mine is just bare copper ground about third of a way down....  so the actual contact area and wire thickness is smaller than it appears.  The size of point will, at maximum, touch, but not fully cover 2 windigs.

A properly made Variac winding will usually have the contact tops of the windings pressed down flat to maintain most of the cross-sectional area, not just filed off halfway through.     That's where the one Chinesium-grade one that I saw had blown up.  Cooked the tops right off of a bunch of windings.

Long time ago, I heard straight from a variac repair technician from Matsunaga, apart from the importance of contact area, also for example at the Matsunaga design, the rectangle shaped tip is slanted just few degrees in order for it to make new contact at the next adjacent wire before leaving the previous contacted winding wire. Photo attached below.

[drussell]

Wow - that's a 5 A unit?

Well, since the only rating I know of for these units is "500VA", that would be about 4A at the nominal voltage, so perhaps typically OK to use up to about 2A, given the way things like this often seem to be rated in Chinese watts (power supplies, speakers, power amplifiers, etc.,) similar to battery capacity rated in Chinese mAh.

I generally understand things like metric <-> imperial conversion factors.  They're typically simple ratios, but things like Chinese watts or VA to standard SI units tends to be more of a sliding scale, generally starting at about 2:1, but it varies. 

[drussell]

Long time ago, I heard straight from a variac repair technician from Matsunaga, apart from the importance of contact area, also for example at the Matsunaga design, the rectangle shaped tip is slanted just few degrees in order for it to make new contact at the next adjacent wire before leaving the previous contacted winding wire.

Yes.  They are rotated noticeably off-straight.

Edit: This is also part of the "resistance-between-windings" thing of the carbon brush instead of it just completely shorting adjacent winding turns.

[Cubdriver]

I just popped the contact out of the Powerstat - it looks to be about 6mm long and 1.5mm wide.



-Pat

[Cubdriver]

Another thing is, see how your windings are plated with something?  Mine is just bare copper ground about third of a way down....  so the actual contact area and wire thickness is smaller than it appears.  The size of point will, at maximum, touch, but not fully cover 2 windigs.

A properly made Variac winding will usually have the contact tops of the windings pressed down flat to maintain most of the cross-sectional area, not just filed off halfway through.     That's where the one Chinesium-grade one that I saw had blown up.  Cooked the tops right off of a bunch of windings.

Long time ago, I heard straight from a variac repair technician from Matsunaga, apart from the importance of contact area, also for example at the Matsunaga design, the rectangle shaped tip is slanted just few degrees in order for it to make new contact at the next adjacent wire before leaving the previous contacted winding wire. Photo attached below.

Interesting.  The brush on the Powerstat is perfectly radial - no angle at all that I can see, while your Matsunaga has an obvious angle relative to the centerline.



-Pat

[drussell]

I actually knew I had a D800 but I didn't associate it with using it for video.  It can be done but some features are quite lacking, like not being able to auto-focus during shooting.  Just few weeks ago, I was thinking of re-re-restarting film photography.

They can be fantastic for video!  What lens do you have on it? 

With a compatible lens, you should be able to put it into continuous auto-focus mode (AF-F) where it runs the lens in constant servo-mode and derives focus from the sensor.

[drussell]

Interesting.  The brush on the Powerstat is perfectly radial - no angle at all that I can see, while your Matsunaga has an obvious angle relative to the centerline.

Yeah, that is definitely a slightly different engineering design setup, the brush contact is much "wider" than the narrower, pointier style that I've always pictured when I thought of a Variac, but if you look in that list from carbonbrush that I posted above, there are obviously a lot of different styles!  I'm no Variac expert! 

I'm sure that David Riddle guy from the earlier links above knows a thing or two about Variac brushes. 

[Ian.M]

https://sound-au.com/articles/variac.htm is worth a read, and his comments on the use of anisotropic carbon brushes to reduce the 'shorted turn' circulating current in the turns bridged by the brush are especially noteworthy.

[Electro Detective]

GE Variac got it right back in the day with dual (duo track?) carbon brushes,

good luck skipping and arcing that setup with the rated load applied, smooth as silk tracking from 0.5v to 270 at full stick 

fwiw use the TooHungLow red devils at 40% of their 'optimistic' label rating and you might be ok         (...... )

[engrguy42]

Cool, thanks.

You sure about the wire diameter? Sure looks bigger than #24.

Anyway I had a couple of variac videos pop up when I went to youtube (apparently their AI whatever noticed I was looking at variacs), and I saw a couple talking about the cheap red Chinese models.

One was from a few months ago, and he unboxed a new one. Turned it on (no load) ran the wiper to max and min and the fuse blew when he got to minimum voltage. Turns out the factory had miswired it, and connected the incoming 120v line to the wiper. So when he turned the dial to minimum it was actually stepping up voltage to some huge value on the primary winding. And I assume the fuse blew when the 120v shorted at the minimum dial setting. And I think the guy's lucky he wasn't touching the wrong thing when it stepped up the line voltage to whatever it went to. He was grabbing the metal case with the thing turned on, and had the front panel open. Geez.

Anyway, I never thought of that one...

Another one from a few years back showed a guy with a similar model, and as he rotated the dial a connected light bulb would flicker, indicating a problem with the wiper/brushes. He talked about presumed low quality of these Chinese products, and took it apart to repair. Though while taking it apart he threw up a text graphic on the screen saying "In all honesty, I've owned this unit for about 10 years, and I've overloaded it quite a bit...". So apparently it was working fine for 10 years and he beat the crap out of it and it the windings needed some cleaning, as well as tightening a loose wiper.

So yeah, be careful with stuff that can hurt you. But I think a blanket dismissal of stuff like this is tough to justify. 

[engrguy42]

I took covers off mine and made some photo and measurements.

Bare wire thickness is 0.505mm
With enamel coating it is 0.573mm
(both measured by Mitutoyo Micrometer)

That means AWG 24 equivalent? 

So the wire is good for 1 amp--if it is copper.  After you are done blowing it up, can you check and see if the wire is copper or CCA?

Huh? Where do you get a 1 amp rating for #24?

Anyway, it looks like we're talking about a device with a 5A fuse (according to the front panel), meaning it's only rated to carry maybe 2-3 amps. That's peanuts. Why the concern over a point contact with the brushes? 

[Electro Detective]

...Why the concern over a point contact with the brushes?

When they skip under full load due to poor contact,

you get a free nano arc welding and or plasma cutting demo 

[engrguy42]

...Why the concern over a point contact with the brushes?

When they skip under full load due to poor contact,

you get a free nano arc welding and or plasma cutting demo 

So what? And why are you assuming poor contact?

[BravoV]

...Why the concern over a point contact with the brushes?

When they skip under full load due to poor contact,

you get a free nano arc welding and or plasma cutting demo 

So what? And why are you assuming poor contact?

Goggle for "make before break" term , its quite popular in electronics.

[engrguy42]

...Why the concern over a point contact with the brushes?

When they skip under full load due to poor contact,

you get a free nano arc welding and or plasma cutting demo 

So what? And why are you assuming poor contact?

Goggle for "make before break" term , its quite popular in electronics.

You're assuming the contact is insufficient to bridge the wires. And we're talking about a mere 2-3amps. There's no arc welding or anything close (it requires like 100 amps or something).

[Electro Detective]

Here's some Youtube videos that tell the story for any disbelievers 

youtube.com/watch?v=PS4UeV8epTU

youtube.com/watch?v=po5Xxl_Syxg

youtube.com/watch?v=qRTKHNoiRQI

youtube.com/watch?v=vsnEMttQC_I



FYI + YMMV: even after repairs and fixits, they are still a future trainwreck waiting to happen down the 'track...'   

[engrguy42]

Here's three Youtube videos that tell the story for any disbelievers 

youtube.com/watch?v=PS4UeV8epTU

youtube.com/watch?v=po5Xxl_Syxg

youtube.com/watch?v=qRTKHNoiRQI


FYI + YMMV: even after repairs and fixits, they are still a future trainwreck waiting to happen down the 'track...'   

The first two are the ones I already referred to. The second one is irrelevant, since it was a repair after 10 years of beating the crap out of it. The first was legit.

Vague generalizations like "trainwreck" without data to support it is fairly useless.

[Electro Detective]

Fair enough, try this one for size > youtube.com/watch?v=vsnEMttQC_I

who loves ya? 

[engrguy42]

Fair enough, try this one for size > youtube.com/watch?v=vsnEMttQC_I

who loves ya? 

The guy replaced the voltmeter with a digital one. So what?

[Electro Detective]

it had a faulty earth ground connection too,
you didn't watch all of it before rushing back to stick in the boots   

come on, there is no denying they are just more TooHungLow dumped junk to money grub frugal or clueless electronics enthusiasts 

and put newb lives at risk with shoddy sweatshoppe alleyway assembly line fails

These red  -Very-Aches-  are not even worthy as door stops 

[engrguy42]

it had a faulty earth ground connection too,
you didn't watch all of it before rushing back to stick in the boots   

come on, there is no denying they are just more TooHungLow dumped junk to money grub frugal or clueless electronics enthusiasts 

and put newb lives at risk with shoddy sweatshoppe alleyway assembly line fails

These Vary-Aches are not even worthy as door stops 

I try to get the facts first before deciding, rather than making up my mind first then looking for stuff to support what I want to believe.

[Electro Detective]

Buy one, open it up carefully, take a look inside, give it a test, and go from there

If the usual suspects don't give you grief to fix, call it a keeper for now

Otherwise find the receipt if bought local, return it re-assembled exactly as you found it,
and let the next guy cop it, to learn the hard way that going too cheap on AC control gear is a NO GO 

[BravoV]

@Electro Detective, just do not easily get provoked. 

[Electro Detective]

Who, cuddly Teddy me ? 

 

[engrguy42]

Buy one, open it up carefully, take a look inside, give it a test, and go from there

If the usual suspects don't give you grief to fix, call it a keeper for now

Otherwise find the receipt if bought local, return it,
and let the next guy cop it, and learn the hard way that going too cheap on such gear is a NO GO 

I 100% agree with the idea of "don't buy junk". However, these devices are so simple that, aside from serious design flaws, getting them working well seems to be pretty simple. You can always point to a "yeah, but...". But this is different from a complex scope, for example, that you can't repair with a quick soldering iron. And the benefit of saving $200+ might outweigh the cost of some minor repairs.

The big question (for me at least) is whether there are any serious design flaws that apply to many/most units. 

[engrguy42]

@Electro Detective, just do not easily get provoked. 

And that should be easy as long as we stick to facts. 

[engrguy42]

it had a faulty earth ground connection too,
you didn't watch all of it before rushing back to stick in the boots   

He meggered the case, and showed that it has good insulation. That's good. Exposed bare metal is bad. He also showed there was a bad connection on the case ground, and repaired it. I'm guessing it took less than 10 minutes to clean/repair the connection.

So at this point I figure the best approach when buying one of these is "Be careful. Check the wiring and case ground first, and if there's a problem you may have to make some simple repairs".

Unless we can find some serious design flaws in many of these units.

[ArthurDent]

On fusing variacs/Powerstats, here are edited snippets of a post I made a couple of years ago in the forums about properly fusing a 240V center tapped variac with 120 V input connected to a center tap of the winding to get 0-240V output as a step-up autotransformer.

“Fuse both the input and output for the current rating of the Powerstat…

 If you have a 10A Powerstat wired for 120 V in and try to get 240V out, the current ratio is 2:1 and the INPUT 10A fuse will blow if you try to get more than 5A out at 240V, there is no way to get more than 5 A out at a 2:1 ratio-period.  The input 10A fuse will protect the Powerstat when set to output more than the input voltage.

If you have this 10A Powerstat wired for 120V in and try to get 12V out, the current ratio is 1:10 and the OUTPUT 10A fuse will blow if you try to get more than 10A out but the input current will only be 1A with 12V at 10A out. If you don't fuse the output and short it, up to 100A will try to flow through the output to cause a 10A current in the input at a 1:10 ratio and the lower end of the variac winding will burn out before the input fuse will blow. The output fuse will protect the Powerstat when set to output voltage less than the input voltage.

The only way to completely protect a Powerstat is to fuse both the input and output for the current rating of the Powerstat.”

[drussell]

However, these devices are so simple that, aside from serious design flaws, getting them working well seems to be pretty simple.
...
The big question (for me at least) is whether there are any serious design flaws that apply to many/most units.

The operating principle of these devices is very simple.  Producing a quality, reliable implementation is not.

As others have suggested since the beginning of this thread, anyone who decides to take the plunge on one of these cheap Chinese "Variac" clones should have a working electrical knowledge and be prepared to thoroughly check their unit own for any possible issues, especially given the history of known defects out of the box and apparent complete lack of QC or testing of final products before shipping.

If the cheap Chinese mechanism is sufficient for your application, great.  Use one.

But to suggest that they are perfectly adequate design, just as good as the "big boys", is nonsense.

[engrguy42]

However, these devices are so simple that, aside from serious design flaws, getting them working well seems to be pretty simple.
...
The big question (for me at least) is whether there are any serious design flaws that apply to many/most units.

The operating principle of these devices is very simple.  Producing a quality, reliable implementation is not.

As others have suggested since the beginning of this thread, anyone who decides to take the plunge on one of these cheap Chinese "Variac" clones should have a working electrical knowledge and be prepared to thoroughly check their unit own for any possible issues, especially given the history of known defects out of the box and apparent complete lack of QC or testing of final products before shipping.

If the cheap Chinese mechanism is sufficient for your application, great.  Use one.

But to suggest that they are perfectly adequate design, just as good as the "big boys", is nonsense.

If they meet their specs, then isn't that an adequate design? Why do you presume these are inadequate designs? I'm just looking for facts, not vague comparisons to "big boys", whatever that is.   

[drussell]

It seems silly to me to buy one of the Chinese ones for $60 when you can get one of these for $60 instead:

https://www.ebay.com/itm/Choice-Powerstat-Autotransformer-Variac-7-1-2-amp/402261290041

I realize the shipping is probably free from Amazon and the ones in that listing are used and look "old" instead of shiny and new, but I'd take an old 7.5A PowerStat over a 2~4A (maybe?) Chinese knock-off any day.

Even if I had to pay for shipping. 

Heck, even if I had to send the brush away to be re-tipped for $25, I would still be happier in the end.  YMMV.

[drussell]

If they meet their specs, then isn't that an adequate design? Why do you presume these are inadequate designs? I'm just looking for facts, not vague comparisons to "big boys", whatever that is.

LOL...  That's just it!

They don't even HAVE specs!

A proper Variac like the 7.5A Powestat models above will be able to do it's full rated output current of 7.5A, even on the very bottom end of the winding.

The Chinese ones don't even tell you how much current they're supposed to be able to deliver.

[engrguy42]

If they meet their specs, then isn't that an adequate design? Why do you presume these are inadequate designs? I'm just looking for facts, not vague comparisons to "big boys", whatever that is.

LOL...  That's just it!

They don't even HAVE specs!

A proper Variac like the 7.5A Powestat models above will be able to do it's full rated output current of 7.5A, even on the very bottom end of the winding.

The Chinese ones don't even tell you how much current they're supposed to be able to deliver.

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

And do you know how old those Powerstats are? I'm guessing 1940's. That company has been around since the 1930's. It's up to you, but buying a device that's been kicked around for maybe 70 years doesn't seem like a good choice to me.

[engrguy42]

BTW, word to the wise...

If you look at the ebay photo of the two Powerstats, the one on the left has magic black smoke all over the plug. 

Don't waste your money on that one. And I'm guessing it's the older one based on that weird black case design. Although maybe good for a museum or something.

[BravoV]

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

Btw, what happened to your other thread that wanting to buy a variac ?

You don't even own a variac, and probably never used one too, let alone publicly admitted you have Dunning Kruger. 

Also, say they labelled it 5 Amp, why derate it just to 2-3 Amps ? Why not just 1 or even 0.5 ?  Just because you said so ? 

[Cubdriver]

If they meet their specs, then isn't that an adequate design? Why do you presume these are inadequate designs? I'm just looking for facts, not vague comparisons to "big boys", whatever that is.

LOL...  That's just it!

They don't even HAVE specs!

A proper Variac like the 7.5A Powestat models above will be able to do it's full rated output current of 7.5A, even on the very bottom end of the winding.

The Chinese ones don't even tell you how much current they're supposed to be able to deliver.

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

And do you know how old those Powerstats are? I'm guessing 1940's. That company has been around since the 1930's. It's up to you, but buying a device that's been kicked around for maybe 70 years doesn't seem like a good choice to me.

So, the 5A fuse means that they're good for 2-3 amps?  Exactly?     2-3A doesn't seem very 'exact' to me - seems a rather large margin, actually.  You buy your cheap but shiny and new far east variac, and I'll stick with old but dull used ones in decent condition with a proven track record and conservative specs that they will meet all day every day.

-Pat

[engrguy42]

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

Btw, what happened to your other thread that wanting to buy a variac ?

You don't even own a variac, and probably never used one too, let alone publicly admitted you have Dunning Kruger. 

Also, say they labelled it 5 Amp, why derate it just to 2-3 Amps ? Why not just 1 or even 0.5 ?  Just because you said so ? 

Ah, so now the personal attacks start.

Anyway, the reason why is because a fuse generally starts thinking about melting at around 50% of its rating. So 50% of 5 amps is 2.5 amps. And based on what I've seen the wire can handle that current, so there should be no problem running it up near 2.5 amps.

And we're back to whether there's some unknown design characteristic we're unaware of that's limiting it below 2.5 amps. Well, if they put in a 5 AMP fuse they're pretty much saying how much it's rated for. And since nobody seems to have any data to prove that it IS limited below 2.5 amps, it's all vague generalizations.

[drussell]

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

And do you know how old those Powerstats are? I'm guessing 1940's. That company has been around since the 1930's. It's up to you, but buying a device that's been kicked around for maybe 70 years doesn't seem like a good choice to me.

Having a fuse holder marked for 5A is not exactly a thorough characterization of device capabilities.

I suggest you take a look at this example 1958 POWERSTAT Catalog to see how the "big boys" have been specifying these things for decades.

You go buy the red Chinese one and post the specifications from the manual, then see how long it lasts.

My money is on that black one on the left outliving your shiny red one by decades, even with a 50+ year head start.      

I'd take that one any day.  Hands down.  No question.

YMMV.

[engrguy42]

If they meet their specs, then isn't that an adequate design? Why do you presume these are inadequate designs? I'm just looking for facts, not vague comparisons to "big boys", whatever that is.

LOL...  That's just it!

They don't even HAVE specs!

A proper Variac like the 7.5A Powestat models above will be able to do it's full rated output current of 7.5A, even on the very bottom end of the winding.

The Chinese ones don't even tell you how much current they're supposed to be able to deliver.

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

And do you know how old those Powerstats are? I'm guessing 1940's. That company has been around since the 1930's. It's up to you, but buying a device that's been kicked around for maybe 70 years doesn't seem like a good choice to me.

So, the 5A fuse means that they're good for 2-3 amps?  Exactly?     2-3A doesn't seem very 'exact' to me - seems a rather large margin, actually.  You buy your cheap but shiny and new far east variac, and I'll stick with old but dull used ones in decent condition with a proven track record and conservative specs that they will meet day all day every day.

-Pat

Come on, this isn't rocket science. You can go to the Littlefuse website, find a 5 amp fuse, and look at the time/current curve. It tells you exactly the time it takes to melt at any current. But anyone with the least amount of electrical knowledge should know that your limitation is around 2.5 amps, and if it's that important you can test it and buy another fuse.

[drussell]

Anyway, the reason why is because a fuse generally starts thinking about melting at around 50% of its rating.

Huh?

That's not how fuse ratings work.

A typical Littelfuse 3AG Fast-Blow fuse is rated to hold 100% of rated current for a minimum of 4 hours continuous.

On the other hand, at 135% of rating it must open in less than 1h maximum.  At 200% it must open in seconds.

The input fuse still has virtually nothing to do with the actual output current rating and allowed load characteristics.

Edit:

But anyone with the least amount of electrical knowledge should know that your limitation is around 2.5 amps

[Cubdriver]

If they meet their specs, then isn't that an adequate design? Why do you presume these are inadequate designs? I'm just looking for facts, not vague comparisons to "big boys", whatever that is.

LOL...  That's just it!

They don't even HAVE specs!

A proper Variac like the 7.5A Powestat models above will be able to do it's full rated output current of 7.5A, even on the very bottom end of the winding.

The Chinese ones don't even tell you how much current they're supposed to be able to deliver.

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

And do you know how old those Powerstats are? I'm guessing 1940's. That company has been around since the 1930's. It's up to you, but buying a device that's been kicked around for maybe 70 years doesn't seem like a good choice to me.

So, the 5A fuse means that they're good for 2-3 amps?  Exactly?     2-3A doesn't seem very 'exact' to me - seems a rather large margin, actually.  You buy your cheap but shiny and new far east variac, and I'll stick with old but dull used ones in decent condition with a proven track record and conservative specs that they will meet day all day every day.

-Pat

Come on, this isn't rocket science. You can go to the Littlefuse website, find a 5 amp fuse, and look at the time/current curve. It tells you exactly the time it takes to melt at any current. But anyone with the least amount of electrical knowledge should know that your limitation is around 2.5 amps, and if it's that important you can test it and buy another fuse.

So to determine the actual output capabilities of my el-cheapo variac, I need to go to the FUSE MANUFACTURER'S website, look up the fuse, and then do calculations?  Seriously?   

<Edit to add - and is the far east built-to-be-as-cheap-as-possible variac going to HAVE a Littelfuse or Bussman fuse that is accurately characterized in it?  Or will it be a Wun Hung Lo brand fuse of who knows what actual rating?>

[bdunham7]

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

And do you know how old those Powerstats are? I'm guessing 1940's. That company has been around since the 1930's. It's up to you, but buying a device that's been kicked around for maybe 70 years doesn't seem like a good choice to me.

Holy carp dude!  Do you have a warehouse full of these things that you're selling?  Your wild guesses are no better than anyone elses here.

First, I'll admit that my estimation of the ampacity of the 24awg winding was off.  I based it on a transformer winding rule-of-thumb of 2.3A/mm^2 and then doubling it because it's a variac.  I looked at two variacs of mine, both Staco, one with 24AWG rated at 3A and one with 26AWG rated at 2.25A.  So reputable manufacturers are using about 6.0A/mm^2.

However, I don't see how you justify your assertion that they tell you exactly that the maximum current is 2-3 amps (as if "2-3 amps" is exact in the first place) especially since it says 500VA on the front as well.  I would argue that that labeling would permit you to connect a 5A load at 100V and let it roast. 

Lastly, how much obvious QC will you ignore in your support of this junk.  Poor solder, wiring internally wrong, not grounded, etc, and you think you can just fix all this (on a brand new unit) and just assume that all of the other things that you can't test or didn't notice will be just fine?  That the enamel insulation is top quality, the iron core is sufficient, etc etc?  When a product or company has severe, obvious issues the first thing I think is "what else did they screw up" not "gee, I can patch that up and I'll have a bargain".  If I'm going to patch something up, I'd prefer a high quality product that has withstood the test of time.

[Cubdriver]

They tell you exactly. There's a big 5AMP fuse label on the front. Which means you're limited to 2-3 amps.

And do you know how old those Powerstats are? I'm guessing 1940's. That company has been around since the 1930's. It's up to you, but buying a device that's been kicked around for maybe 70 years doesn't seem like a good choice to me.

Holy carp dude!  Do you have a warehouse full of these things that you're selling?  Your wild guesses are no better than anyone elses here.

First, I'll admit that my estimation of the ampacity of the 24awg winding was off.  I based it on a transformer winding rule-of-thumb of 2.3A/mm^2 and then doubling it because it's a variac.  I looked at two variacs of mine, both Staco, one with 24AWG rated at 3A and one with 26AWG rated at 2.25A.  So reputable manufacturers are using about 6.0A/mm^2.

However, I don't see how you justify your assertion that they tell you exactly that the maximum current is 2-3 amps (as if "2-3 amps" is exact in the first place) especially since it says 500VA on the front as well.  I would argue that that labeling would permit you to connect a 5A load at 100V and let it roast. 

Lastly, how much obvious QC will you ignore in your support of this junk.  Poor solder, wiring internally wrong, not grounded, etc, and you think you can just fix all this (on a brand new unit) and just assume that all of the other things that you can't test or didn't notice will be just fine?  That the enamel insulation is top quality, the iron core is sufficient, etc etc?  When a product or company has severe, obvious issues the first thing I think is "what else did they screw up" not "gee, I can patch that up and I'll have a bargain".  If I'm going to patch something up, I'd prefer a high quality product that has withstood the test of time.

He seems to like taking contrarian positions then digging in hard.

https://www.eevblog.com/forum/testgear/good-power-supply/msg3019052/#msg3019052

https://www.eevblog.com/forum/testgear/atx-power-supplies/msg3025932/#msg3025932

-Pat

[engrguy42]

bdunham, I'm not supporting anything. I don't give a flying F@@@ what anyone buys. But for some reason you guys can't comprehend that I'm looking for facts. I don't have personal grudges, I'm not a fanboy for anything, I'm just doing what engineers do: sort thru the personal bias and unsupported BS generalizations and find the facts.

Apparently that type of approach pisses people off here because it challenges you to put up or shut up. And since you can't provide data you resort to personal attacks.

Fine. When somebody can provide facts they're worth listening to. Otherwise it's just the typical muscle flexing to act smart. 

[tkamiya]

Guys?

This is why I volunteered to test this thing.  Too much theorizing over things we don't understand 100%.  Everyone knows bits and pieces. There's theory and there's what works in practice.  Has everyone seen very large slidac like 3 phase 200 amp job?  I have.  It was motorized.  Then had a fun discussion with an old instructor. 

Today, I'm feeling very poorly.  I don't know if I can get it done today but I am curious.  I wanna see this thing spark and burn up.  I have 3 fire extinguishers loaded on a cart.  Face shield is ready. 

Someone asked me about the lens.  I have 24-70 2.8, 70-210 2.8, 150mm 2.8 macro, 8mm fish eye, etc, etc, etc.  One of the deficiencies of D800 was focus while shooting.  That's what D810 solves.  I think.  I never used it for video.  This is the first time.

[tkamiya]

By the way, I did study fuse and breakers in depth when I was in trade school.  5A fuse will pass 5A.  But good practice is if you need to pass 5A, better use bigger fuse.  Breakers work little differently.  You need to pass 5A, you use 5A.  But usual usage needs to be 70 to 80% of that.  Otherwise you have to up-size everything.  I can dig up an old text book with rating curve and all that but I'm not going to translate it.  So start studying Japanese 

This thread is starting to resemble a lug nut thread on automotive forum.

[drussell]

He seems to like taking contrarian positions then digging in hard.

Indeed, all the while talking about only wanting the facts. 

It seems that he is one to like to latch onto whatever "facts" support his particular preconceived position, like if a Variac says it should have a 5A fuse in it, that must mean it is rated for an output of "2-3A" (is that resistive only?  who knows), while essentially dismissing any and all evidence to the contrary.

I'm now trying out the user-ignore feature for some serenity. 

[engrguy42]

By the way, I did study fuse and breakers in depth when I was in trade school.  5A fuse will pass 5A.  But good practice is if you need to pass 5A, better use bigger fuse.  Breakers work little differently.  You need to pass 5A, you use 5A.  But usual usage needs to be 70 to 80% of that.  Otherwise you have to up-size everything.  I can dig up an old text book with rating curve and all that but I'm not going to translate it.  So start studying Japanese 

This thread is starting to resemble a lug nut thread on automotive forum.

Yes, that was my mistake. I was thinking larger power fuses. I can give examples but the whole discussion is irrelevant.

[Cubdriver]

bdunham, I'm not supporting anything. I don't give a flying F@@@ what anyone buys. But for some reason you guys can't comprehend that I'm looking for facts. I don't have personal grudges, I'm not a fanboy for anything, I'm just doing what engineers do: sort thru the personal bias and unsupported BS generalizations and find the facts.

Apparently that type of approach pisses people off here because it challenges you to put up or shut up. And since you can't provide data you resort to personal attacks.

Fine. When somebody can provide facts they're worth listening to. Otherwise it's just the typical muscle flexing to act smart.

So where's the data for the shiny red variac?  Someone posted a catalog from Superior Electric showing the specs of the Powerstats.  Current ratings, temperature rise, construction points, connection diagrams  That's data that, as an engineer, I can look at and be confident in.  On the other hand, the shiny red variac has???  The "data' we're providing is that the established brands actually PUBLISH data, and the cheapies do not.  How do I compare data to no data?  If I'm buying something and manufacturer A provides me a data sheet showing characteristics, and option B is something on amazon that is sold by multiple outlets with a possible name change or different colored case and some sort of vague specs that it *might* meet, well, to me the choice is pretty obvious.

-Pat

[bdunham7]

bdunham, I'm not supporting anything. I don't give a flying F@@@ what anyone buys. But for some reason you guys can't comprehend that I'm looking for facts. I don't have personal grudges, I'm not a fanboy for anything, I'm just doing what engineers do: sort thru the personal bias and unsupported BS generalizations and find the facts.

Apparently that type of approach pisses people off here because it challenges you to put up or shut up. And since you can't provide data you resort to personal attacks.

Fine. When somebody can provide facts they're worth listening to. Otherwise it's just the typical muscle flexing to act smart.

What qualifies as a 'fact' for you?  Are the conclusions you have asserted supported by such facts?

[drussell]

Someone asked me about the lens.  I have 24-70 2.8, 70-210 2.8, 150mm 2.8 macro, 8mm fish eye, etc, etc, etc.  One of the deficiencies of D800 was focus while shooting.  That's what D810 solves.  I think.  I never used it for video.  This is the first time.

With a D or G series lens installed (and possibly other newer ones, I don't know, and their are proper dedicated cine lenses too, of course) you should theoretically have options in the Auto-Focus setup while you're in Movie mode to have the AF configured between face-priority, wide-area AF, normal-area AF, subject-tracking, full-manual, etc.

What does it list for possible options in the Auto-Focus setup, in Movie mode, with the lens you're using installed?

Barring that, are you possibly running very old firmware, is there perhaps a firmware update?

[engrguy42]

Guys?

This is why I volunteered to test this thing.  Too much theorizing over things we don't understand 100%.  Everyone knows bits and pieces. There's theory and there's what works in practice.  Has everyone seen very large slidac like 3 phase 200 amp job?  I have.  It was motorized.  Then had a fun discussion with an old instructor. 

Today, I'm feeling very poorly.  I don't know if I can get it done today but I am curious.  I wanna see this thing spark and burn up.  I have 3 fire extinguishers loaded on a cart.  Face shield is ready. 

Someone asked me about the lens.  I have 24-70 2.8, 70-210 2.8, 150mm 2.8 macro, 8mm fish eye, etc, etc, etc.  One of the deficiencies of D800 was focus while shooting.  That's what D810 solves.  I think.  I never used it for video.  This is the first time.

Sorry to hear you're not feeling well. Don't bother with the testing. Sadly, I think this is yet another situation where people believe what they want to believe, and facts are irrelevant. I'm thinking I might also buy one and do my own testing to see what the actual design limitations are, if any. More data is better. 

[Cubdriver]

Buy one, open it up carefully, take a look inside, give it a test, and go from there

If the usual suspects don't give you grief to fix, call it a keeper for now

Otherwise find the receipt if bought local, return it,
and let the next guy cop it, and learn the hard way that going too cheap on such gear is a NO GO 

I 100% agree with the idea of "don't buy junk". However, these devices are so simple that, aside from serious design flaws, getting them working well seems to be pretty simple. You can always point to a "yeah, but...". But this is different from a complex scope, for example, that you can't repair with a quick soldering iron. And the benefit of saving $200+ might outweigh the cost of some minor repairs.

The big question (for me at least) is whether there are any serious design flaws that apply to many/most units.

Reading back a little bit, this one struck me.  Let's go with your assertion that "these devices are so simple".  If they're 'so simple', then why can't the MANUFACTURER make them right?  Why should someone NEED to open up their brand new shiny variac to make sure that it's grounded properly, or otherwise wired correctly?  They're such simple devices, you'd think the place that makes them could do it.  The big question to ME is that if the place that builds them can't consistently PUT THEM TOGETHER CORRECTLY, how can I be confident that they are designed properly?  Looking at the photos put up earlier, it appears to me that the Powerstat I posted has a brush with about 4x the contact area as the conical one shown on the red variac.  The Powerstat is rated at 3A and the instructions are to fuse it at that level.  The red one is fused at 5A.  Is the red one adequately designed and Superior Electric so super conservative that they used 4x brush size for 3/5 of the current, or is the red one running right at the edge?  As an engineer, look at the two and tell me which one YOU think will be more reliable in the long term.

-Pat

[tkamiya]

Starting with slightly modified process.  I'll be back shortly.  Having issue with video.

[engrguy42]

Damn...4 thermocouples. Excellent !!!

Just be careful. I know you have a fire extinguisher, but be careful. Think 3 times before doing anything 

[engrguy42]

BTW, I'm curious what folks are expecting for temperatures. What constitutes a "fail"?

[drussell]

BTW, I'm curious what folks are expecting for temperatures. What constitutes a "fail"?

We're looking for data, not a pass/fail go/no-go test.

We would like to know things like core temperature rise after running at a given load long enough to stabilize, etc. so that it can be compared to the known data from other units' known, published specifications.  eg. those 1958 PowerStat specs state there will be no more than 50°C core temperature rise when operated at maximum ratings (which is maximum current down low or the total maximum allowed VA, depending on wiper position), ambient temperature -20 to +40°C, with a published de-rating curve for operation above 40°C ambient.

etc.

[Shock]

My moneys on the Fluke surviving.

[engrguy42]

BTW, I'm curious what folks are expecting for temperatures. What constitutes a "fail"?

We're looking for data, not a pass/fail go/no-go test.

We would like to know things like core temperature rise after running at a given load long enough to stabilize, etc. so that it can be compared to the known data from other units' known, published specifications.  eg. those 1958 PowerStat specs state there will be no more than 50°C core temperature rise when operated at maximum ratings (which is maximum current down low or the total maximum allowed VA, depending on wiper position), ambient temperature -20 to +40°C, with a published de-rating curve for operation above 40°C ambient.

etc.

So it seems like the goal is to end up saying "See, the old Powerstat is better because it has lower temperatures"? I'm not sure how, or to what extent, a comparison to other equipment is relevant.

To me it seems the question is "Can these cheap Chinese variacs supply around the rated 5Amps/500VA continuously without overheating and causing internal damage" or something like that. 

[tkamiya]

OK, done. 

Summary:
Environment is outside in Florida 90 to 100 degrees F depending on if sun is out or not.
Total run time 10 minutes
50V 2.4amp for 8 minutes or so
120V 4.2amp for rest of the test
Highest 120V 4.2A = 504VA = 504watts
Core temperature stayed around 100 degrees F.
Hottest part is 137 F.  The big nut at bottom which holds the spindle.
Side and top is somewhere between the two.
No spark, nothing melting, no smoke.

At this time, I don't see any reason to continue so ended the test around 15 minutes.
Messing with video right now but do you guys really want to see this?  Absolutely nothing happens.

[engrguy42]

OK, done. 

Summary:
Environment is outside in Florida 90 to 100 degrees F depending on if sun is out or not.
Total run time 10 minutes
50V 2.4amp for 8 minutes or so
120V 4.2amp for rest of the test
Highest 120V 4.2A = 504VA = 504watts
Core temperature stayed around 100 degrees F.
Hottest part is 137 F.  The big nut at bottom which holds the spindle.
Side and top is somewhere between the two.
No spark, nothing melting, no smoke.

At this time, I don't see any reason to continue so ended the test around 15 minutes.
Messing with video right now but do you guys really want to see this?  Absolutely nothing happens.

Good stuff. Thanks. I'm kinda surprised the temps are so low (137F/58C) , but I suppose with the outdoor breeze maybe that helped.

[drussell]

OK, done. 

Summary:
Environment is outside in Florida 90 to 100 degrees F depending on if sun is out or not.
Total run time 10 minutes
50V 2.4amp for 8 minutes or so
120V 4.2amp for rest of the test
Highest 120V 4.2A = 504VA = 504watts
Core temperature stayed around 100 degrees F.
Hottest part is 137 F.  The big nut at bottom which holds the spindle.
Side and top is somewhere between the two.
No spark, nothing melting, no smoke.

Well, at 120v it is just going straight through....  so that's kinda pointless to check

The highest voltage drop should be around 60v and also at the highest boost setting.

The biggest question is can you actually pull 4 or 5 amps out of it at those "tougher" settings, where it will be dissipating the most, or right at the bottom of the range, etc. 

You will need additional load than just the single bulb to test that, though....

Edit:



I would like to know how that tiny brush behaves when you crank it back and forth with 4 or 5 amps through it, too.

[engrguy42]

drussell, are you concerned about some resistive dissipation in the windings causing higher temps?

[tkamiya]

LCD is just plain awful outside.  Can't read a thing!  So I don't think I'll upload this.  Is that ok with you guys?  All you get is for 16 minutes worth of what I just summarized on previous post.

Yes, breeze definitely helped.  But high ambient temp did not.  So this is not a conclusive test.  There is no way I'd do this inside or in a box of some kind. 

As to how to interpret this, I am really not sure.  I am still not comfortable leaving it unattended.  I think any slidack by anyone will see some rise in temp.  Was it excessive?  I don't think so.  I think it was reasonable.  How it was constructed leaves me with a lot of reservations.  So from here, unless someone does more test and more accurate test, it will just be personal feelings.

So end result is:
I'm alive.
My house did not burn down.
Fuse survived.
No sparks.
Moderate temp rise.

[drussell]

drussell, are you concerned about some resistive dissipation in the windings causing higher temps?

Well, at Vout=Vin, the windings are dissipating their lowest possible amount, so, yeah. 

[tkamiya]

I did crank it few times up and down for a minute or so.  Nothing happened.

Do you guys want me to quickly test with 1000 watts bulb at 50V? 

[bdunham7]

OK, done. 

Summary:
Environment is outside in Florida 90 to 100 degrees F depending on if sun is out or not.
Total run time 10 minutes
50V 2.4amp for 8 minutes or so
120V 4.2amp for rest of the test
Highest 120V 4.2A = 504VA = 504watts
Core temperature stayed around 100 degrees F.
Hottest part is 137 F.  The big nut at bottom which holds the spindle.
Side and top is somewhere between the two.
No spark, nothing melting, no smoke.

At this time, I don't see any reason to continue so ended the test around 15 minutes.
Messing with video right now but do you guys really want to see this?  Absolutely nothing happens.

It's up to you, but variacs are often used in quasi-continuous service.  Can you do 100 volts for an hour?

[drussell]

Yeah, I would love to see what happens on some longer tests at closer to a full current rating for the wiper setting that are more than a few percent away from "straight through."

If, for example, you're troubleshooting an SMPS, it is going to TRY to pull more current as you lower the voltage, so will an induction motor load, etc., so that's what I'm most interested in. 

Right at 120v I'm sure it can do the 4.1A / 500VA just fine.

[engrguy42]

drussell, are you concerned about some resistive dissipation in the windings causing higher temps?

Well, at Vout=Vin, the windings are dissipating their lowest possible amount, so, yeah. 

I'm just ballparking that maybe 24AWG wire has say 5 ohms per 1,000 ft (just a guess), and say there's, I dunno, 200 ft wire in the variac, that's only like an ohm. At 5 amps that's dissipating only another 25 watts or so? Doesn't seem like it would do much, but maybe my numbers are off.

My basic problem with this is we're talking only 5 amps in this big hunk of copper. I can't imagine big damaging temperatures, unless I'm missing something.

[tkamiya]

Did a quick test with 1000 watt bulb.  I couldn't go up to 50V.  At 27V, it is already drawing 4.9A.  No appreciable temperature raise in 2 minutes or so.  What I did notice was I felt more HUM on the knob. 

[bdunham7]

I'm just ballparking that maybe 24AWG wire has say 5 ohms per 1,000 ft (just a guess), and say there's, I dunno, 200 ft wire in the variac, that's only like an ohm. At 5 amps that's dissipating only another 25 watts or so? Doesn't seem like it would do much, but maybe my numbers are off.

My basic problem with this is we're talking only 5 amps in this big hunk of copper. I can't imagine big damaging temperatures, unless I'm missing something.

More like 25 ohms/1000ft and I've no idea how long the wire is, but the way to test this would be to measure the voltage drop directly by comparing OC to loaded, which Tkamiya seems to be well equipped to do.  Based on how much adjustment it takes to keep my similarly-sized variac at the correct voltage when the load changes, I'm betting the dissipation is not negligible.

EDIT:  Or you could use an ohmmeter.  My particular unit has two inputs, both 120V but one labeled 50/60Hz, which is 14 ohms, the other 60Hz, which is 12 ohms.  This is the 2.25A/26awg model.

[Electro Fan]

snip

Today, I'm feeling very poorly.  I don't know if I can get it done today but I am curious.  I wanna see this thing spark and burn up.  I have 3 fire extinguishers loaded on a cart.  Face shield is ready. 

Someone asked me about the lens.  I have 24-70 2.8, 70-210 2.8, 150mm 2.8 macro, 8mm fish eye, etc, etc, etc.  One of the deficiencies of D800 was focus while shooting.  That's what D810 solves.  I think.  I never used it for video.  This is the first time.

tkamiya,

Sorry to hear if you aren’t feeling well but it looks like you are having some more good finds.

Thanks for being a good referee and for volunteering some DUT gear and time to experiment.  Hope you have enough fire extinguishers and some good PPE - but I’d bet if needed somehow you could find some more.   

Seriously, keep us posted on your tests and stay safe.   

[tkamiya]

I really wish I had a better load.  All I have is 500 watt bulb and 1000 watt bulb.  Well, I also have 40 watt bulb but that's nothing. 

It was rather boring test, so I had to keep reminding myself of "potential danger."  500 watts can cause significant injury and destruction.

I'll keep the setup so if anyone finds better ways to test with what I have, please let me know.

[engrguy42]

I'm just ballparking that maybe 24AWG wire has say 5 ohms per 1,000 ft (just a guess), and say there's, I dunno, 200 ft wire in the variac, that's only like an ohm. At 5 amps that's dissipating only another 25 watts or so? Doesn't seem like it would do much, but maybe my numbers are off.

My basic problem with this is we're talking only 5 amps in this big hunk of copper. I can't imagine big damaging temperatures, unless I'm missing something.

More like 25 ohms/1000ft and I've no idea how long the wire is, but the way to test this would be to measure the voltage drop directly by comparing OC to loaded, which Tkamiya seems to be well equipped to do.  Based on how much adjustment it takes to keep my similarly-sized variac at the correct voltage when the load changes, I'm betting the dissipation is not negligible.

I found an online copper wire resistance calculator and assuming a 200ft length of 0.5 mm wire it comes up with 5 ohms. If each turn is 1 foot long, then that's 200 turns? Seems reasonable? Maybe 1.2 volts per turn accuracy?

[tkamiya]

snip

Today, I'm feeling very poorly.  I don't know if I can get it done today but I am curious.  I wanna see this thing spark and burn up.  I have 3 fire extinguishers loaded on a cart.  Face shield is ready. 

Someone asked me about the lens.  I have 24-70 2.8, 70-210 2.8, 150mm 2.8 macro, 8mm fish eye, etc, etc, etc.  One of the deficiencies of D800 was focus while shooting.  That's what D810 solves.  I think.  I never used it for video.  This is the first time.

tkamiya,

Sorry to hear if you aren’t feeling well but it looks like you are having some more good finds.

Thanks for being a good referee and for volunteering some DUT gear and time to experiment.  Hope you have enough fire extinguishers and some good PPE - but I’d bet if needed somehow you could find some more.   

Seriously, keep us posted on your tests and stay safe.   

Thanks.  I used to be in photography.  I was never in video.

During the test, I had a face shield on and fire extinguisher handy.  I also had a quick way to disconnect the power source. 

[engrguy42]

snip

Today, I'm feeling very poorly.  I don't know if I can get it done today but I am curious.  I wanna see this thing spark and burn up.  I have 3 fire extinguishers loaded on a cart.  Face shield is ready. 

Someone asked me about the lens.  I have 24-70 2.8, 70-210 2.8, 150mm 2.8 macro, 8mm fish eye, etc, etc, etc.  One of the deficiencies of D800 was focus while shooting.  That's what D810 solves.  I think.  I never used it for video.  This is the first time.

tkamiya,

Sorry to hear if you aren’t feeling well but it looks like you are having some more good finds.

Thanks for being a good referee and for volunteering some DUT gear and time to experiment.  Hope you have enough fire extinguishers and some good PPE - but I’d bet if needed somehow you could find some more.   

Seriously, keep us posted on your tests and stay safe.   

Thanks.  I used to be in photography.  I was never in video.

During the test, I had a face shield on and fire extinguisher handy.  I also had a quick way to disconnect the power source.

Thanks much tkamiya. Greatly appreciated. Since I'm not really sure what specific issues people have with these devices in terms of poor/cheap/junky performance I'm not sure what else you can test.

Like I say I think it would be tough to under-design a 5 amp device like this. Now if we're talking say 50 amps that's a different story.

[drussell]

I'll keep the setup so if anyone finds better ways to test with what I have, please let me know.

Ironically, you need a second Variac to be able to vary your bulb-based load bank to test the first Variac. 

You don't happen to have any giant wirewound adjustable resistors or anything like that, eh? 

[bdunham7]

I really wish I had a better load.  All I have is 500 watt bulb and 1000 watt bulb.  Well, I also have 40 watt bulb but that's nothing. 

It was rather boring test, so I had to keep reminding myself of "potential danger."  500 watts can cause significant injury and destruction.

I'll keep the setup so if anyone finds better ways to test with what I have, please let me know.

I think that the time periods are short as there is a significant thermal mass.  It sounds like you have better things to do and you don't want to leave it unattended, but I would do the test for several hours at a reasonable load--say 25V/4A with the 1000 watt bulb or 100V/3A with the 500--or find a small space heater or toaster to get a constant resistance.

[ArthurDent]

Well, at Vout=Vin, the windings are dissipating their lowest possible amount, so, yeah. 

That is very true but you still have to fuse the variac properly, and some people don't. Years ago I was given a nice bench Powerstat where someone hadn't fused the output. The input went to a tap to allow the output to be higher than the input and the wiper was just a couple of turns from the 100% tap when the output was shorted and burned the two turns between the tap and the wiper. I epoxied a small formed copper link in place of the burned turns and soldered the two good ends of the coil to the tabs I formed on the end of the piece. I ended up with an almost new bench variac with a very narrow band where the voltage didn't increase for a degree or two of rotation. As I said before these burned turns are generally at the low end of the winding but could be repaired the same way.

[bdunham7]

I found an online copper wire resistance calculator and assuming a 200ft length of 0.5 mm wire it comes up with 5 ohms. If each turn is 1 foot long, then that's 200 turns? Seems reasonable? Maybe 1.2 volts per turn accuracy?

Yes, 5 ohms for 200 feet, not 1000.  But I can't see a turn being a foot long on such a small device--but it likely has more than 200 turns.  But those are guesses, not facts.  An ohmmeter will actually answer the question.

[Electro Fan]

My moneys on the Fluke surviving.

+1 for the Fluke surviving but also...

My moneys on two additional things:  1) this debate will go on beyond the first test regardless of what comes from the first test, and 2) tkamiya will find some more good stuff if he decides to do further tests

[drussell]

That is very true but you still have to fuse the variac properly, and some people don't.

Indeed.

The red Chinese ones only have an input fuse, so anyone who uses one should really add an output fuse holder when they do their wiring check/repair/update.  This also makes it very easy to put in an appropriate output fuse for the device you're working on when needed.

[Electro Fan]

I'll keep the setup so if anyone finds better ways to test with what I have, please let me know.

Ironically, you need a second Variac to be able to vary your bulb-based load bank to test the first Variac. 

You don't happen to have any giant wirewound adjustable resistors or anything like that, eh? 

Challenging a champion finder to find something?

[BravoV]

Ironically, you need a second Variac to be able to vary your bulb-based load bank to test the first Variac. 

Pssttt .... 

Too bad I don't have the time, as I have IR cam and curious too.

Don't feel guilty as I'm TEA god worshiper.   

[engrguy42]

I've had my eye on a 20A version of the cheap red Chinese variacs, and happened on a guy's video of the exact same model. If anyone is curious about what the internals look like in real good light, here it is:



He did what I would do and replace the high-side 20A fuse with a 15 amp since my bench circuit is on a 15 amp breaker. Otherwise the internals look reasonable.

My only concern is it's a big, 16 pound monster and I worry about shipping from whereever. Heck, I'm not even sure I need that many ampules. I can't imagine what I have that would use more than 20 amps on the low side. Or even 10. I dunno, maybe I'll just get a 10 amp one.

Oh, and here's what they give for specs on Amazon. Interesting they mention 8 working hours:

Max. 2000VA
Plug Type: US Plug
Max output : 20 Amps
Input Voltage: AC 110V/60Hz
Output Voltage: AC 0-130V/60Hz
Material: Iron (shell) & Copper Coil
Humidity : ≤ 5%
Working Hours: 8 hours
Condition Temperature:<40℃
Insulated Resistance: >10MΩ
Withstand Voltage: 2KV/1 minute

[Electro Detective]

This comment has been scanned and verified to ensure no 'personal attack' content or sneaky Auto-Troll browser addons I may be unaware of 

Let's have a brief gander at those specs above:

"Withstand Voltage: 2KV/1 minute"

what kind of 2KV are they specifying?

A hand held insulation tester at 2.5KV pumping out an awesome current of 2mA DC through the red shed paint powder coating? ? 


Another curious spec above states "Working Hours: 8 hours"

Is there a timer on board to indicate when the unit is at its 'end of 8 hour life' and time for another purchase?
Sounds expensive unless you only use it for 8 minutes every year, then sell it off at 4 hours and buy another new one 

 
Giggles aside     "Working Hours: 8 hours" tells us nothing about its true duty cycle with any load   

and "Condition Temperature: < 40℃"  and some/all of the other click buyer eye candy techie specs above, tell even less


If I just had no choice but to buy one of these  fire red  Very-Aches (ED 'Patented' term), I'd roll with a dissected, corrected and in/out tightly fused 20 amp job,
and use it under 5 amps max full load continuous, with crossed fingers confidence,

and not leave the bench unattended in case Nature calls
and stuck on the throne royale way longer than anticipated   

[tkamiya]

I'll keep the setup so if anyone finds better ways to test with what I have, please let me know.

Ironically, you need a second Variac to be able to vary your bulb-based load bank to test the first Variac. 

You don't happen to have any giant wirewound adjustable resistors or anything like that, eh? 

Challenging a champion finder to find something?

A second variac?  I found one!  On Amazon!!

[tkamiya]

I suppose, I could start with 500 watt bulb, and keep adding 100 watt bulbs or less until I reach the target?  You guys want 50V 4A, correct? 

I *could* do that.

What are we expecting to see?  At this point I'm fairly confident the thing will survive.  What's the "junk" criteria?

[engrguy42]

I suppose, I could start with 500 watt bulb, and keep adding 100 watt bulbs or less until I reach the target?  You guys want 50V 4A, correct? 

I *could* do that.

What are we expecting to see?  At this point I'm fairly confident the thing will survive.  What's the "junk" criteria?

Wow. Excellent question !!! 

Can't wait to hear the answer

 

[tkamiya]

It is getting dark so I had to cut the test short.

I'm using 4 amp as load max because of 500VA and input is 120V.  Also, fuse is 5 amp.  I did go up to 4.5 amp or so and nothing worth mentioning happened.

500 watts + 150 watts bulbs made 4.0 amps at 66.6V.
During 5 minutes run, all temperatures stayed nominal.
Core is 82F
Bottom screw is 78F
Side is 84F (where the contact cone is)
Top is 80F

Ambient is 76F with slight breeze.
Each temperature, including core, fluctuated quite a bit with presence of some wind.  (above figure is when air is still)
Nothing melted
No sparks
No abnormal heating of cone was observed
Turn-it-up-and-down caused not even a small spark

Now, here's something...  a FAINT a VERY FAINT burning smell was identified but it was unclear where it was coming from.  It could be the bulb as they were dusty.  Tried to smell around the stat area, but I couldn't find the source.

Another thing to note, temperature was significantly low compared to day time test.  That means temperature RISE (delta) may be critical.  Doing the same test in a sweaty warm warehouse with no breeze may cause something.  (remember, this is Florida)

Long time test is probably warranted, but I really can't sit outside for that long, and with children around, leaving it unattended is just plain irresponsible.  As we push this harder and harder, something will happen at some point.  Thanks to someone's post of specs, it is obvious this is meant for a light duty job.  It was never meant to be used continuously at max rating in harsh environment.

I'll leave you with a photo of a test supervisor. 

[Electro Fan]

I'll keep the setup so if anyone finds better ways to test with what I have, please let me know.

Ironically, you need a second Variac to be able to vary your bulb-based load bank to test the first Variac. 

You don't happen to have any giant wirewound adjustable resistors or anything like that, eh? 

Challenging a champion finder to find something?

A second variac?  I found one!  On Amazon!!

There was never a doubt 

[rsjsouza]

I didn't read the whole thread, but years ago I bought a Mastech 2kVA and I still have it. It is a workhorse!

https://www.eevblog.com/forum/buysellwanted/mastechvolteq-variac-comments-on-quality/

[cdev]

That old equipment photographed so well almost has a "fetishistic" quality to it now. Its almost erotic.

Fwiw, my variac looks a lot like this plus two meters and two fuses and it looked like it just came new (inside and out) from the factory.

https://www.ebay.com/p/1127332453?iid=372842692756

Call the owner in that eBay link above that I first posted above (I don't know anything about the unit or seller in this link just above in this post).


Edit: Ok, here you go
http://www.shitintogold.com/VARIAC.php
http://www.shitintogold.com/

Another Edit: Owner's name is David.  Looks like he might have a different/new URL; I think the company name was / maybe still is DRCO.  I might have picked a different URL but you're buying a variac not a PR firm.

Yet another edit:
Tell David you don't want (can't afford / can't justify) the brand new cosmetics - you just want his least expensive reliable variac and see what he says.  His pricing reflects the fact that he likes to make products that operate like and look like they came new from GenRad decades ago - but I think he can cut the price down if you go for the electronics over the case condition.  Tell him that you are going to report back to EEVblog if he takes good care of you (that's his only MO anyway) and therefore this is his chance to tap into a new market - but first you need a reliable variac that you can afford.  If his least expensive unit is too expensive for your budget keep shopping elsewhere - but I think he will try to take good care of you.

Good luck!

And another PS,

Those Powerstats like Shock posted look good.  Ask David if you go for a Powerstat vs. a GenRad if that would help keep the budget in line.  For all I know the Powerstats might be the same or more or less; the good thing with David is that he just likes making and refurbing good products and he's big into variacs.  I don't think he has any axe to grind whether it's Powerstat, GenRad, Staco, or any other manufacturer, model, or design.  It's like asking Michelangelo for something in marble - but it needs to be an affordable statue.

[Shock]

I took covers off mine and made some photo and measurements.

Take a look at your variacs chassis earth/ground wire (green/yellow) and see if it's making proper physical metal to metal contact to all the chassis metalwork parts. It looks like the lug is sandwiched between two painted surfaces and the front panel is just screwed in. I see it also has the wrong mains cable coloring so the plug and socket could be wired and switched backwards for the US. The soldered connections look a bit suspect as well, did they pass them through the lugs or is it just tacked in place?

[tkamiya]

Good eyes!

Yes, it's a lug sandwiched between painted surfaces.  Inner side metal is tapped, and lug is making an imperfect contact.  I actually can align it perfectly concentric and tighten, and have no ground connection to the outer casings.  The lug is soldered on the back side, and properly.  All other solder connections are suspects

US color is black for live, white for neutral, and green or bare for ground.  Color coding does not appear to be consistent.  It is obviously not US or Japanese standard colors.  Maybe EU colors, mostly?  Polarity is correct.  I visually followed it.  Personally, I am not too strict on coloring wires, if it doesn't confuse myself.

I found one case of awful soldering.  This is at the main switch.  Multiple wires are there because this is its main neutral connection.  Those are DEEP cracks, gentlemen!  It appears to be cracks that formed after soldering was completed.  Looks like it was pulled a part under force.  Very dull in color indicating cold solder job, too.  This will need to be redone.

Also note, black char marks on insulation.  I think these jackets are vinyl.  I would expect at least PVC.  (or cloth covered rubber on old ones)

[tkamiya]

So....  what would YOU do to this if you owned this?  Obviously, it isn't completely useless or junk.  It functions.  Also, obviously, in current state, it's unsafe.

I would at least replace all wires with properly rated ones and redo soldering jobs.  Ground connection will be upgraded.  I'd use it at 50% duty rating.  Remove the meter and replace as it is pretty much useless.  I really would like to re-case it.  Even mine, a smaller one, is very flimsy.  I will also take it completely apart and make sure ground to live isolation is good and sound.  One thing I don't have an ability to check is winding/live to case isolation.  As this is an auto-transformer, it's critical that casing never becomes live.

[BravoV]

So....  what would YOU do to this if you owned this?  Obviously, it isn't completely useless or junk.  It functions.  Also, obviously, in current state, it's unsafe.

I would at least replace all wires with properly rated ones and redo soldering jobs.  Ground connection will be upgraded.  I'd use it at 50% duty rating.  Remove the meter and replace as it is pretty much useless.  I really would like to re-case it.  Even mine, a smaller one, is very flimsy.  I will also take it completely apart and make sure ground to live isolation is good and sound.  One thing I don't have an ability to check is winding/live to case isolation.  As this is an auto-transformer, it's critical that casing never becomes live.

Yep, tear it apart and inspect, as have friend own similar model, and the locking and securing mechanism of the heavy toroid is not properly done, and quite dangerous.

Probably they didn't fasten enough and the toroid maybe shifted or moved not centered inside the round case/chassis during shipment.

[tkamiya]

I took it apart completely.  There were few surprises.  I'm sorry, no pictures.

The shaft that starts in the knob, goes though the casing, the core (the transformer) and finally secured at bottom is not insulated anywhere.  There are fiber washers but nothing is protecting from touching the case. 

The cone shaped brush is secured to a brass plate which is secured to plastic that is secured to the shaft.  A wire goes from brush to outside after wrapping around the shaft twice.

What this means is that if anything energized touches the shaft, the whole thing will become energized.  The only ground connection to the casing is iffy.  (see my last post).  The core itself is secured by a large piece of rubber and metal disk like any toroidal transformer would be.

I would like some redundancy in protection here.  For example, use of silicone wire with abrasion protection, and some kind of insulating sleeve on the main shaft where it goes through the core.

Another surprise is, since this is a transformer, there is a beginning of the winding and the end of the winding.  This is hiding inside the core.  Inside, there is a piece of fiber washer, probably M4.  Each end of the wire goes through this once and each is loosely twisted to itself few times.  There is no means of positively securing this from comping apart.  Also, beginning and end of the wire is within 3mm of each other.  There is maximum voltage differential here.  Here, I would like to see potting, clamps, or some type of secure method.  Also, more distance.

I gave it more thought on if this is a junk or not.  It's not a junk because it functions.  It is a product that is made just enough to pass the requirement.  But every measure of cost cutting has been made to meet the price point.  That's why we couldn't find clear faults or catastrophic failures.  But, there is no redundancy for safety.  There is no robustness.  Safety seemed to have took a second seat to the price.  It works.  It sells.  It makes profit.  That's the objective.

To me, I was asking the wrong question.  I should have been asking, is it safe?  is it robust?  is it quality?  Is it junk is purely a personal opinion based on personal standard.  I have mine but it's different from everyone on this board.

That concludes my involvement in this experiment.  I've cut the cord, and placed pieces in the recycle bin.  That was a fun and educational experience for me.  Thanks everyone for the encouragement.

[Shock]

Looks like you didn't keep it but I'll leave this here anyway as I wrote it out.

As BravoV mentioned you need to look closely at the mechanical side of things. When I brought my Powerstat variac secondhand I had to do this, they aren't inherently safe as people tend to mess with them or they get abused or knocked around in shipping.

In your case the soldered connections need to be mechanically attached to the lug plus soldered so if the joint breaks it won't come off. Some strain relief like glue lined heat shrink and using cable ties to prevent wires flopping about will help prevent failures. The front panel needs a protective earth strap connected to it. Polarity of the plug needs to be verified to the switch and fuse.

I'm not worried about the wiring color but on cheap Chinese products you need to check it all thoroughly. See if it's a solid copper strand power cable and what current the cable is rated for, 240V cables have a typically lower current rating than 120V. I suspect the fuse may not be rated properly either, can also be fake of course.

Long and narrow brushes are the optimal shape for variacs, it allows a more reliable electrical and mechanical connection to the winding and prevents excessive interwinding shorts. The more windings shorted the less current handling and greater unnecessary heat. Something like that anyway.

[Ian.M]

tkamiya: "I've cut the cord, and placed pieces in the recycle bin."
That says it all.   Not worth fixing up or impractical to make safe enough to use, so pretty much the dictionary definition of 'junk'. The only way it could be worse and still function would be if it had an intentionally live shaft extending up into the knob!

I was surprised to hear it didn't even have a slip-ring to connect to the brush.  Pity you didn't take pictures during the final disassembly - they would have made a nice horror-show to point cheap-skate newbs at . . .

[Cubdriver]

tkamiya - thank you for the autopsy report.  That definitely sounds like it was designed to the lowest price point.

-Pat

[Electro Detective]

so basically as mentioned before here and other threads      these are another TooHungLow item UNSAFE and or DANGEROUS to run out of the box without checking the item first,
as well as be very familiar with variable auto transformers, so you know what to look for and sort out.
Yes they look simple enough to wire up (famous last words?) and even simpler to stuff up,
especially if it wasn't done right at the factory, usually a polarity mix up and a suspect earth ground to chassis connection.

fwiw the stepdown transformers from TooHungLow I've seen and had to make safe/R, but not any more ever again! the asses that import such junk to Australia should be tar and feathered on Youtube,
and off to prison for trading in dangerous goods, and false/forged/fake quality control check labeling.       

I blame the dirtbag sellers for dumping cheap half baked risk laden electronic rubbish on to cash strapped newbs, bargain hunters, and unsuspecting cheapskates,
not the clueless whipped workers in China   

How hard can it be to show the impoverished slave workers on the sweatshoppe assembly lines how to do it properly,
and pay them 1c more per hour, so they don't complain about the extra effort to pay attention and understand what it is they are assembling and why some details are IMPORTANT,
and sellers can flog a safer decent cheap crap item, and stitch the end connedsumer for $10 more

[engrguy42]

Thanks much, tkamiya. Greatly appreciated. As I suspected, there were no results that would be sufficient for the "it's cheap, red, Chinese junk" folks to ever change their minds. They'll always find something. People believe what they want to believe.

For a bunch of hobbyists to push back on something like this is hard to comprehend. Folks who spend hours and days building and restoring things can't accept a 10 minute review of wiring and maybe a little re-soldering in order to save $200? Tech experts who don't know enough to do a few tweaks (which may be unnecessary in the first place)? But hell, you'll buy a 70 year old something on eBay and spend hours cleaning and restoring it and that's fine. Or spend days designing,prototyping,soldering and testing components on PCB's. But instead of "wow, look what I found on Amazon for only $50!!! Only took a few minutes of tweaking and it works great!!", we get "Well, *I* own a Powerstat/Variac/whatever and anything that isn't a Powerstat is junk". 

Anyway, nothing new here. BTW, my ATX power supply is working great.   

[Electro Detective]

..the "it's cheap, red, Chinese junk" folks to ever change their minds. They'll always find something. People believe what they want to believe.

I always 'find something' wrong with these fire red russian roulette devices, and other stepdown transformer junk with wonky windings and internals from Wee R Sweatshop Inc. 

in fact it's a winning bet there's ALWAYS some drama with these, or will be in the future if not sorted 

I 'want to believe' what I've seen for real, too many times, and rectified,
so users don't zap their attached gear and or themselves
or their curious bench hopping cats having a sniff

[Gyro]

Thanks much, tkamiya. Greatly appreciated. As I suspected, there were no results that would be sufficient for the "it's cheap, red, Chinese junk" folks to ever change their minds. They'll always find something. People believe what they want to believe.

I hardly think that having a brand new off the shelf item and needing to dismantle and modify it to make it safe to use, counts as "They'll always find something". 

What about all the purchasers who don't know that they need to do that (assuming they have the skills)?

[Shock]

As I suspected, there were no results that would be sufficient for the "it's cheap, red, Chinese junk" folks to ever change their minds.

Go buy one then if you think it's great quality. You deserve to own one of these.

[engrguy42]

For those who might also be considering one of these cheap Chinese pieces of junk that might kill your cat, I did find a 2000VA, 20 A model on Amazon that looks identical to the others. But this comes with a digital voltage display rather than the fairly useless analog one. I'm sure the display will require some reference adjustment or whatever. And this one is only $83 with free shipping (maybe that's an Amazon Prime thing?). And that's good cuz it's 15 pounds.

And this one has a spec that says "Temperature Rise: <60℃". I'm still on the fence about whether I'll get one (since honestly I don't really need one), but I would like to learn more about them, and actually test one myself. So I can judge based on facts.

Which, BTW, I'd encourage others to consider, so that you can judge based on facts and your own tests, rather than unsupported, vague generalities.

Kinda like I did with the ATX power supplies. In spite of being bombarded by unsupported, vague generalities and name calling, I actually built one (modified my 460 watt ATX supply) and now it works great. I use it all the time. Although someone said those ATX supplies are bad because a bug can crawl in and zap the power supply, so I'm still worrying about that. 

[ArthurDent]

Post 145 listing cheap variac specs:

I've had my eye on a 20A version of the cheap red Chinese variacs, and happened on a guy's video of the exact same model. If anyone is curious about
Oh, and here's what they give for specs on Amazon. Interesting they mention 8 working hours:

Max. 2000VA
Plug Type: US Plug
Max output : 20 Amps
Input Voltage: AC 110V/60Hz
Output Voltage: AC 0-130V/60Hz
Material: Iron (shell) & Copper Coil
Humidity : ≤ 5%
Working Hours: 8 hours
Condition Temperature:<40℃
Insulated Resistance: >10MΩ
Withstand Voltage: 2KV/1 minute

...so is 8 working hours the life expectancy? 

[ArthurDent]

I took covers off mine and made some photo and measurements.

Take a look at your variacs chassis earth/ground wire (green/yellow) and see if it's making proper physical metal to metal contact to all the chassis metalwork parts. It looks like the lug is sandwiched between two painted surfaces and the front panel is just screwed in. I see it also has the wrong mains cable coloring so the plug and socket could be wired and switched backwards for the US. The soldered connections look a bit suspect as well, did they pass them through the lugs or is it just tacked in place?

There is a big difference between an older well-made variac and a new one built to be as cheap as possible.

For one thing, my Powerstat was designed for industrial use. The set screws that hold the shaft in place can be loosened to allow the base of the Powerstat to be mounted on a panel with the shaft sticking out what you’d think of as the bottom of the unit, not the top. This shaft is not connected to any live parts of the Powerstat. To allow this mounting, what was the zero output position when viewed from the top, is now max output when viewed from the bottom because of the reversed rotation when the variac is flipped like this. This means that the taps on the winding have to be symmetrical so the unit can be used with either rotation and these connections are shown in one of the attached photos.

The narrow width of the brushes on a variac has been mentioned several times but the brush has to be wide enough to bridge at least 2 turns to prevent arcing. The graphite brush is resistive and doesn’t directly short any turns. You can see how the brush in mine ‘bridges’, not shorts, turns on the coil. If you count the number of turns in a variac and divide the input voltage by the number of turns you will find that there is a fraction of a volt between turns that will be distributed across the brush surface. That is why you don’t see metal brushs used in variacs because they would actually short out adjacent turns.

On this Powerstat all the wires connected to the coil are brought out to a panel where your connections are made. You can see in the photo below where the connections are symmetrical to allow reverse rotation depending on how the Powerstat is mounted.

Where this was an older bench variac, it had a 2 wire cord and 2 wire outlet which I changed. As I mentioned in a previous post I added a second fuse holder on the output wiper to properly protect it. As has been mentioned previously you can also put a smaller rated fuse in the output if you only have a small load you want to limit the maximum current to. I also added a 3mm LED to give me a visual indication that the power was on.  I had to be careful when I added this extra ‘stuff’ but there was just enough room to allow everything to fit nicely.

If you check the rating plate you will see that this is actually a 230 VAC 3.5 Amp Powerstat but it works fine on 120 VAC, perhaps even running a little cooler.   

[Cubdriver]

Post 145 listing cheap variac specs:

I've had my eye on a 20A version of the cheap red Chinese variacs, and happened on a guy's video of the exact same model. If anyone is curious about
Oh, and here's what they give for specs on Amazon. Interesting they mention 8 working hours:

Max. 2000VA
Plug Type: US Plug
Max output : 20 Amps
Input Voltage: AC 110V/60Hz
Output Voltage: AC 0-130V/60Hz
Material: Iron (shell) & Copper Coil
Humidity : ≤ 5%
Working Hours: 8 hours
Condition Temperature:<40℃
Insulated Resistance: >10MΩ
Withstand Voltage: 2KV/1 minute

...so is 8 working hours the life expectancy? 

I was wondering what that meant, too.  For something that small, it seems an odd number for a duty cycle - it would certainly reach max temp in far less than 8 hours, and if it WERE temp related it should be indicated and have some operational limit below which it can run continuously.  Things like arc welders are typically rated at their max output with a ten minute window (say, 20% at max power,  meaning you can weld for two minutes out of every ten if you're running it full bitch, and a continuous rating that might be around 40% of max, at which you can operate continuously, and a curve between the two for the intermediate values - perhaps at 80% power you can go 40% - 4 minutes out of every ten.)

I guess that's asking too much from a red scariac that you need to rework right out of the box.

-Pat

[Electro Detective]

Hey, I spotted the 'Working Hours: 8 hours' thing ages ago 

Note the other specs are just as informative for discerning spendthrifts like me

So if no real new, used or abused variable AC units were around anymore on Earth,
I would chew the bullet and go for a red 20 amp unit, if 5 continuous amps is actually what I needed

opening up and securing the internal snafus first..

and buy 2 units if anticipating a long 16 hour session at the workbench
or if one decides on a life change as a toaster

 

[tkamiya]

It really sounds like a case of something written in Chinese was auto-translated or manually translated by someone without technical knowledge.  I used to have to back translate and re-translate manuals originated from Japan.

It probably means it only works from 9am to 5pm with mandatory lunch break at noon.  Overtime is at reduced rate only.      I think, somehow, it means non-continuous duty.  But I have no idea what the de-rating curve is.  I wonder if someone can get a hold of the original Chinese manual?  I know we have members here from China....

Humidity of less than 5%?  I have no idea what this means.  These things are usually fine at non-condensating environment.  It may have meant 95% or less. 

[cdev]

I basically have two unused, "new old stock" US made, very small variacs - just the raw variacs, no case. They are just the raw autotransformer, no additional hardware. I forget the brand, they are buried deep in a closet. Probably the wattage is okay for some very small electronics, but nothing of any size. I will get them out and photograph the winding of the one that is open and post it here.

[Cubdriver]

I basically have two unused, "new old stock" US made, very small variacs - just the raw variacs, no case. They are just the raw autotransformer, no additional hardware. I forget the brand, they are buried deep in a closet. Probably the wattage is okay for some very small electronics, but nothing of any size. I will get them out and photograph the winding of the one that is open and post it here.

I'll save you the trouble.  I'm pretty sure I have a few little ones fairly readily accessible.  Could be a fun macro project.

-Pat

[cdev]

I have been meaning to test what the highest frequency that coulld go through them is. I wonder if perhaps one could be turned into a "PHAT" variable inductor for RF? And incorporated into an antenna tuner, maybe for VLF?

[Cubdriver]

I have been meaning to test what the highest frequency that coulld go through them is. I wonder if perhaps one could be turned into a "PHAT" variable inductor for RF? And incorporated into an antenna tuner, maybe for VLF?

My guess as to highest frequency would be 'not very' - remember, these are meant to be efficient at line voltages and frequencies - I bet they get lossy fast if you start going higher.  I just grabbed some I had and am taking some photos - one is a French-made GenRad that came out of an old Balzers e-gun supply; it's rated for 2 A @ 220 Vac, 50-400 Hz.  I doubt any of them are meant to operate much above that, and that even going to 400 Hz would be uncommon unless it's something geared towards military and/or aircraft use.I suspect that even the 3 kHz bottom end of the VLF band would be too high.

-Pat

[Electro Detective]

Sounds like a plan guys 

I might one day break out the bits and pump some audio frequencies in mine with an amp and sig gen starting at 50hz and go from there

-----------------

fwiw guys: one major design flaw with most if not all variacs IMVHO, and I've seen a few made in the various countries that produce goodies that work beyond 8 hours   
is if the carbon brushes snap or wear down to nothing,
the metal housing comes crashing down and destroying the copper windings

The variac may still work at certain ranges due to direct brass or steel to copper contact grinding along   
till the operator opens it up to investigate why the unit drops out sometimes, or running warm 

Some plastic or isolating material placed just before the 'end of life' point on the carbon brush assembly should stop that from happening,
and alert the operator it's new carbon brush time when the variac starts skipping and arcing/sizzling, or not working at all.

Its a better bet than trashed copper windings which is a pain to fix/maybe,
and I will be rigging up something simple and functional to my humble threesome of variable 240 volt AC units, on the next open and inspect/clean session

Anything is possible with a junk box of 'not ready for landfill yet' scraps, bits and bobs, glue, cable/zip ties, sharp knife,  rotary tool
and bottomless cups of tea etc 

Have a look next time you're in there guys, you'll know what I mean about it,
it may bite you one day or not,
so why not step in and avoid that, with an easy knockup ugly prevention fix no one will see or care,

rather than forced to replace a decent trashed unit with a clickey clickey online shiny red one,
that's apparently only good for 7 hours and 59 minutes..   

 

[drussell]

I have been meaning to test what the highest frequency that coulld go through them is. I wonder if perhaps one could be turned into a "PHAT" variable inductor for RF? And incorporated into an antenna tuner, maybe for VLF?

Certainly won't go anywhere close to up into the RF range, given the physical dimensions, but might be useful for something-or-other in some sort of project.  The '58 POWERSTAT catalog has the following curve, indicating that those would at least be fine for the 400 Hz frequency used for things like aircraft power:

[cdev]

What's inside the toroid coil, what kind of magnetics do they use in Variacs??

[ArthurDent]

What's inside the toroid coil, what kind of magnetics do they use in Variacs??

It's basically the same type of steel used in a conventional transformer wound up into a coil like a roll of paper towels then coated with something like epoxy to insulate it from the wire wound on it.

https://3.imimg.com/data3/KX/GP/MY-2618182/variac-toroidal-core-500x500.jpg

[cdev]

Ahhh.. so basically impossible to take out and replace with something else.. LOL, oh well..